If there is probable cause, a citizen can be arrested even without a warrant. However, probable cause is a tricky concept because there is not a precise definition. Yet, in situations where there is a warrantless arrest, the courts judge the facts on an objective standard. More specifically, the court asks if the facts available to the officer at the moment of the seizure or the search "warrant a man reasonable caution in the belief" that the action taken was appropriate? Carroll v. U.S., 267 U.S. 132, 162 (U.S. 1925). If not, the officer may have intruded upon the citizen's constitutionally guaranteed rights because the search or seizure was "based on nothing more substantial than inarticulate hunches." Terry v. Ohio, 392 U.S. 1, 22 (U.S. 1968).
The legality of searches and seizures is not based on an officer's subjective motivations but on the objectively reasonable basis for the search or seizure. On the one hand, a minor traffic violation can justify a defendant's custodial arrest even though the officer actually stopped the defendant to investigate some other unlawful conduct. On the other hand, there is no probable cause to arrest a defendant even if the search reveals the defendant's alleged participation in illegal activities when the officer does not observe any illegal behavior, and when the officer does not receive information from a reliable source. Lawrence v. Georgia, 686 S.E.2d. 352, 354-55 (Ga. App. 2009).
While sometimes there is no probable cause and your constitutional rights have been violated, this is a complex topic for which a lawyer is needed to determine whether your rights have actually been violated. An incorrect determination could lead to an additional charge of resisting or evading arrest. To avoid this costly misunderstanding, you should speak to a lawyer immediately upon arrest to be sure that you are within your constitutionally guaranteed rights.