delivered the opinion of the Court.

The question presented by this case is whether an arrest made in good-faith reliance on an ordinance, which at the time had not been declared unconstitutional, is valid regardless of a subsequent judicial determination of its unconstitutionality.

I

At approximately 10 p. m. on September 14, 1976, Detroit police officers on duty in a patrol car received a radio call to investigate two persons reportedly appearing to be intoxicated in an alley. When they arrived at the alley, they found respondent and a young woman. The woman was in the process of lowering her slacks. One of the officers asked what they were doing, and the woman replied that she was about to reheve herself. The officer then asked respondent for identification; respondent asserted that he was Sergeant Mash, of the Detroit Police Department; he also purported to give his badge number, but the officer was unable to hear it. When respondent again was asked for identification, he changed his answer and said either that he worked for or that he knew Sergeant Mash. Respondent did not appear to be intoxicated.

Section 39-1-52.3 of the Code of the City of Detroit provides that a police officer may stop and question an individual if he has reasonable cause to believe that the individual’s behavior warrants further investigation for criminal activity. In 1976 the Detroit Common Council amended § 39-1-52.3 to provide that it should be unlawful for any person stopped pursuant thereto to refuse to identify himself and produce evidence of his identity.^[1]

[*34] When he failed to identify himself, respondent was taken into custody for violation of § 39-1-52.3;^[2] he was searched by one of the officers who found a package of marihuana in one of respondent’s shirt pockets, and a tinfoil packet secreted inside a cigarette package in the other. The tinfoil packet subsequently was opened at the station; an analysis established that it contained phencyclidine, another controlled substance.

Respondent was charged with possession of the controlled substance phencyclidine. At the preliminary examination, he moved to suppress the evidence obtained in the search following the arrest; the trial court denied the motion. The Michigan Court of Appeals allowed an interlocutory appeal and reversed. It held that the Detroit ordinance, § 39-1-52.3, was unconstitutionally vague and concluded that since respondent had been arrested pursuant to that, ordinance, both the arrest and the search were invalid.

The court expressly rejected the contention that an arrest made in good-faith reliance on a presumptively valid ordinance is valid regardless of whether the ordinance subsequently is declared unconstitutional. Accordingly, the Michigan Court of Appeals remanded with instructions to suppress the evi[*35]dence and quash the information. 80 Mich. App. 197, 262 N. W. 2d 921 (1977).

The Michigan Supreme Court denied leave to appeal. We granted certiorari, 439 U. S. 816 (1978), to review the Michigan court’s holding that evidence should be suppressed on federal constitutional grounds, although it was obtained as a result of an arrest pursuant to a presumptively valid ordinance. That holding was contrary to the holdings of the United States Court of Appeals for the Fifth Circuit that such arrests are valid. See United States v. Carden, 529 F. 2d 443 (1976); United States v. Kilgen, 445 F. 2d 287 (1971).

II

Respondent was not charged with or tried for violation of the Detroit ordinance. The State contends that because of the violation of the ordinance, i. e., refusal to identify himself, which respondent committed in the presence of the officers, respondent was subject to a valid arrest. The search that followed being incidental to that arrest, the State argues that it was equally valid and the drugs found should not have been suppressed. Respondent contends that since the ordinance which he was arrested for violating has been found unconstitutionally vague on its face, the arrest and search were invalid as violative of his rights under the Fourth and Fourteenth Amendments. Accordingly, he contends the drugs found in the search were correctly suppressed.

Under the Fourth and Fourteenth Amendments, an arresting officer may, without a warrant, search a person validly arrested. United States v. Robinson, 414 U. S. 218 (1973); Gustafson v. Florida, 414 U. S. 260 (1973). The constitutionality of a search incident to an arrest does not depend on whether there is any indication that the person arrested possesses weapons or evidence. The fact of a lawful arrest, standing alone, authorizes a search. United States v. Robinson, supra, at 235. Here the officer effected the arrest of respond[*36]ent for his refusal to identify himself; contraband drugs were found as a result of the search of respondent’s person incidental to that arrest. If the arrest was valid when made, the search was valid and the illegal drugs are admissible in evidence.

Whether an officer is authorized to make an arrest ordinarily depends, in the first instance, on state law. Ker v. California, 374 U. S. 23, 37 (1963); Johnson v. United States, 333 U. S. 10, 15, and n. 5 (1948). Respondent does not contend, however, that the arrest was not authorized by Michigan law. See Mich. Comp. Laws § 764.15 (1970). His sole contention is that since the arrest was for allegedly violating a Detroit ordinance later held unconstitutional, the search was likewise invalid.

Ill

It is not disputed that the Constitution permits an officer to arrest a suspect without a warrant if there is probable cause to believe that the suspect has committed or is committing an offense. Adams v. Williams, 407 U. S. 143, 148-149 (1972); Beck v. Ohio, 379 U. S. 89, 91 (1964). The validity of the arrest does not depend on whether the suspect actually committed a crime; the mere fact that the suspect is later acquitted of the offense for which he is arrested is irrelevant to the validity of the arrest. We have made clear that the kinds and degree of proof and the procedural requirements necessary for a conviction are not prerequisites to a valid arrest. See Gerstein v. Pugh, 420 U. S. 103, 119-123 (1975); Brinegar v. United States, 338 U. S. 160, 174-176 (1949).

When the officer arrested respondent, he had abundant probable cause to believe that respondent’s conduct violated the terms of the ordinance. The ordinance provides that a person commits an offense if (a) an officer has reasonable cause to believe that given behavior warrants further investigation, (b) the officer stops him, and (c) the suspect refuses to identify himself. The offense is then complete.

[*37] Respondent’s presence with a woman, in the circumstances described, in an alley at 10 p. m. was clearly, in the words of the ordinance, “behavior . . . warrant [ing] further investigation.” Respondent’s inconsistent and evasive responses to the officer’s request that he identify himself, stating first that he was Sergeant Mash of the Detroit Police Department and then that he worked for or knew Sergeant Mash, constituted a refusal by respondent to identify himself as the ordinance required. Assuming, arguendo, that a person may not constitutionally be required to answer questions put by an officer in some circumstances, the false identification violated the plain language of the Detroit ordinance.

The remaining question, then, is whether, in these circumstances, it can be said that the officer lacked probable cause to believe that the conduct he observed and the words spoken constituted a violation of law simply because he should have known the ordinance was invalid and would be judicially declared unconstitutional. The answer is clearly negative.

This Court repeatedly has explained that “probable cause” to justify an arrest means facts and circumstances within the officer’s knowledge that are sufficient to warrant a prudent person, or one of reasonable caution, in believing, in the circumstances shown, that the suspect has committed, is committing, or is about to commit an offense. See Gerstein v. Pugh, supra, at 111; Adams v. Williams, supra, at 148; Beck v. Ohio, supra, at 91; Draper v. United States, 358 U. S. 307, 313 (1959); Brinegar v. United States, supra, at 175-176; Carroll v. United States, 267 U. S. 132, 162 (1925).

On this record there was abundant probable cause to satisfy the constitutional prerequisite for an arrest. At that time, of course, there was no controlling precedent that this ordinance was or was not constitutional, and hence the conduct observed violated a presumptively valid ordinance. A prudent officer, in the course of determining whether respondent had committed an offense under all the circumstances shown[*38] by this record, should not have been required to anticipate that a court would later hold the ordinance unconstitutional.

Police are charged to enforce laws until and unless they are declared unconstitutional. The enactment of a law forecloses speculation by enforcement officers concerning its constitutionality — with the possible exception of a law so grossly and flagrantly unconstitutional that any person of reasonable prudence would be bound to see its flaws. Society would be ill-served if its police officers took it upon themselves to determine which laws are and which are not constitutionally entitled to enforcement.

In Pierson v. Ray, 386 U. S. 547 (1967), persons who had been arrested for violating a statute later declared unconstitutional by this Court sought damages for false arrest under state law and for violation of the Fourteenth Amendment under 42 U. S. C. § 1983. Mr. Chief Justice Warren speaking for the Court, in holding that police action based on a presumptively valid law was subject to a valid defense of good faith, observed: “A policeman’s lot is not so unhappy that he must choose between being charged with dereliction of duty if he does not arrest when he has probable cause, and being mulcted' in damages if he does.” 386 U. S., at 555. The Court held that “the defense of good faith and probable cause, which the Court of Appeals found available to the officers in the common-law action for false arrest and imprisonment, is also available to them in the action under § 1983.” Id., at 557. Here, the police were not required to risk “being charged with dereliction of duty if [they did] not arrest when [they had] probable cause” on the basis of the conduct observed.^[3]

[*39] IV

We have held that the exclusionary rule required suppression of evidence obtained in searches carried out pursuant to statutes, not previously declared unconstitutional, which purported to authorize the searches in question without probable cause and without a valid warrant. See, e. g., Torres v. Puerto Rico, 442 U. S. 465 (1979); Almeida-Sanchez v. United States, 413 U. S. 266 (1973); Sibron v. New York, 392 U. S. 40 (1968); Berger v. New York, 388 U. S. 41 (1967). Our holding today is not inconsistent with these decisions; the statutes involved in those cases bore a different relationship to the challenged searches than did the Detroit ordinance to respondent’s arrest and search.

Those decisions involved statutes which, by their own terms, authorized searches under circumstances which did not satisfy the traditional warrant and probable-cause requirements of the Fourth Amendment. For example, in Almeida-Sanchez v. United States, supra, we held invalid a search pursuant to a federal statute which authorized the Border Patrol to search any vehicle within a “reasonable distance” of the border, without a warrant or probable cause. The Attorney General, by regulation, fixed 100 miles as a “reasonable distance” from the border. 413 U. S., at 268. We held a search so distant from the point of entry was unreasonable under the Constitution. In Berger v. New York we struck down a statute authorizing searches under warrants which did not “particularly describ[e] the place to be searched, and the persons or things to be seized,” as required by the Fourth and Fourteenth Amendments. 388 U. S., at 55-56.

In contrast, the ordinance here declared it a misdemeanor for one stopped for “investigation” to “refuse to identify himself” ; it did not directly authorize the arrest or search.^[4] Once[*40] respondent refused to identify himself as the presumptively valid ordinance required, the officer had probable cause to believe respondent was committing an offense in his presence, and Michigan's general arrest statute, Mich. Comp. Laws § 764.15 (1970), authorized the arrest of respondent, independent of the ordinance. The search which followed was valid because it was incidental to that arrest. The ordinance is relevant to the validity of the arrest and search only as it pertains to the “facts and circumstances” we hold constituted probable cause for arrest.

The subsequently determined invalidity of the Detroit ordinance on vagueness grounds does not undermine the validity of the arrest made for violation of that ordinance, and the evidence discovered in the search of respondent should not have been suppressed. Accordingly, the case is remanded for further proceedings not inconsistent with this opinion.

Reversed and remanded.