Citizens' Powers of Arrest
Citizens' arrests are rarely seen, but they do occur. A citizen cannot arrest except in the following cases (in most states):
Felonies or breach of the peace committed within her presence or view; in some states, citizens can arrest with reasonable suspicion
With a warrant naming the citizen as the arrester of a specific person(s)
Prevention of theft and its consequences
Aiding a law enforcement officer or someone authorized to arrest
In all cases, the arrested person must be delivered to law enforcement immediately; otherwise, these actions may be seen as kidnapping. The PI's arrest powers are the same as those of the private citizen's.
If you arrest outside of these parameters, you may be arrested yourself. Some states even allow the arrestee to sue the arresting citizen for false arrest or imprisonment. However, check your state laws because many cases of citizens' arrests have been upheld.
PIs do not normally need to be concerned with arresting or detaining suspects. They are primarily information gatherers. However, if someone is in danger or if evidence is in danger of being destroyed, PIs have been known to detain or even arrest suspects. Most of the time it's better to call the police and let them do the job they're trained to do, yet in those rare cases where you may feel compelled to help someone or protect evidence, the citizen's arrest is legal.
One recent case of citizens' arrest concerned a driver weaving all over a busy freeway, barely missing several vehicles. A young man in a truck slowly eased this erratic driver toward the shoulder of the road. Another driver spotted his efforts, moved in front of the suspect vehicle, and slowed his speed. The two men stopped the car and detained the inebriated driver, claiming citizens' arrest until police arrived, at which time they were praised for possibly saving lives.
Though their actions were brave, they were also dangerous, so don't try this unless you've had training. Most opportunities for citizens' arrest will prove dangerous because the citizen doesn't carry the same authority as a police officer, and they will usually face resistance. Nevertheless, many citizens have stepped in to help others, especially when violence or potential violence is involved.