If the police didn't have probable cause to arrest you and they found incriminating evidence as a result of their encounter with you, the prosecution may argue that you were never under arrest. Otherwise, you could have a strong argument that the evidence the officers found is inadmissible as the result of an illegal arrest.
(For information on the justification officers need for detentions and arrests, see What’s the difference between an arrest and a detention or “stop and frisk”?)
In many instances, it’s clear that someone has been arrested—the officers’ use of handcuffs and their words indicate as much. But the police can also arrest people without formally announcing that they’ve done so.
An arrest occurs when the police take someone into custody. People are in custody when they aren’t free to leave, whether or not the police take them to the police station or jail, use handcuffs, or even announce that an arrest has occurred. The question is whether the police control the suspect’s movement.
Level of Restraint
The use of force—for example, grabbing and handcuffing—is a common way to complete an arrest. But force isn’t necessary; any action by police officers that shows an intent to take the subject into custody and causes the subject to submit to them accomplishes an arrest. In general, whether there’s been an arrest depends on whether a reasonable person in the suspect’s situation would have felt significantly restrained. (When the police have restrained someone to a lesser extent, then the interaction may constitute a detention rather than an arrest. See Arrest vs. Detention: How to Tell Whether You’ve Been Arrested or Simply Detained.)
Example: Officers Carver and Hauk are on patrol, travelling around town in a police car. Wallace notices their vehicle and begins to run. Carver and Hauk follow him; they don’t use a siren or flashers, they don’t issue commands, and they don’t display weapons or block or control his movement. They catch up with him and drive alongside him very briefly, at which point they see him throw several baggies into some nearby bushes. Wallace stops running, and the officers get out of their vehicle. They quickly determine that the baggies contain cocaine, so they then formally arrest Wallace. Wallace wasn’t under arrest before he threw away the baggies; Carver and Hauk’s brief acceleration to catch up to him and driving next to him wouldn’t have led a reasonable person to believe that the officers were trying to capture or stop him. The result is that Wallace doesn't have a defense on the basis that he was illegally arrested. (Michigan v. Chesternut, 486 U.S. 567 (1988).)
For further reading on this topic, see What Is an Arrest?