Differences between arrest with and arrest without a warrant
The Fourth Amendment of the Constitution provides for general rights that aim to protect individuals from exposure to unreasonable arrests, searches or seizures by police offers or other government agents.
However, the specific rules that must be followed by law enforcement are not stated in the body of the Constitution. Rather, the rule of law as it pertains to arrests and search warrants has been created through precedent or Supreme Court case decisions and opinions. The general remedy for an unlawful arrest is to suppress evidence or incriminating statements that were obtained as a result of the arrest from being presented at trial.
A warrant is required if police must enter private property in order to make an arrest or perform a search. Private property consists of the suspect's home or other property that is not open to the public. A warrant can only be acquired if an officer has probable cause. Probable cause exists if there are enough facts or circumstantial evidence to allow a reasonable person to conclude the suspect in question committed a crime. Evidence of probable cause must be presented to a district justice or magistrate judge who confirms the existence of probable cause and issues the warrant. If an officer enters private property to make an arrest or seize evidence minus a warrant when one is required, all evidence acquired will be considered "fruit of the poisonous tree" and will not be permitted as evidence at trial.
Generally, an officer may make an arrest in a public place without a warrant if probable cause exists. Probable cause in this scenario does not require the approval of a judge or magistrate, rather it may be derived from reliable tips, a routine vehicle stop, a "stop and frisk" or any additional evidence that an officer observes. Also, an officer may enter private property to make an arrest, but only if "exigent" or emergency circumstances exist, such as the safety of others or even the suspect himself.
Felony vs. Misdemeanour Crimes
An officer may arrest a suspect without a warrant if he has reasonable grounds to believe the suspect committed a felony grade-crime. It is not necessary that the officer actually see the felony occur in order to make a warrantless arrest. However, in the case of misdemeanour crimes, the officer must have first-hand knowledge the suspect committed the crime in order to make an arrest without a warrant. This means that the misdemeanour must have been committed in the officer's presence.