Citizen's arrest

Penal Code section 837

A private person may arrest another:

1. For a public offense committed or attempted in his presence.
2. When the person arrested has committed a felony, although not in his presence.
3. When a felony has been in fact committed, and he has reasonable cause for believing the person arrested to have committed it.

Secondary sources

From http://blog.ceb.com/2012/06/08/youre-under-a-citizens-arrest/

First off, a citizen’s arrest in California is not limited to citizens, but extends to private persons generally. It is, however, narrower than the authority of police officers and has different search rules.

Citizen’s arrests differ from police arrests in two significant respects, each requiring a higher degree of certainty before a citizen can make a legal arrest: 1.A peace officer can arrest a person on probable cause to believe that the suspect committed a felony (Pen C §836(a)(3)), but a valid citizen’s arrest additionally requires that “a felony has been in fact committed.” Pen C §837(3). This latter requirement can only be met if the person making the arrest has evidence of the corpus delicti before the arrest and it is an offense known by the arresting party to have been committed. People v Aldapa (1971) 17 CA3d 184, 188, 94 CR 579. 2.Unlike a peace officer, a private person may arrest for a misdemeanor only if it’s actually committed or attempted in his or her presence (Pen C §837(1)); “a reasonable belief that a misdemeanor has been committed or attempted is insufficient.” People v Lacey (1973) 30 CA3d 170, 175, 105 CR 72.

There are also different rules on a search incident to a citizen’s arrest. A private person who makes an arrest “is authorized only” to search the arrestee for weapons, “not to conduct a search for contraband ‘incidental’ to the arrest, or to seize such contraband upon uncovering it.” People v Sandoval (1966) 65 C2d 303, 311 n5, 54 CR 123; Pen C §846. But this principle is of little help in a criminal prosecution to a defendant who’s been illegally arrested or searched by a private person because the Fourth Amendment and the exclusionary rule based on it are generally applicable only to searches and seizures by the government or its agents, not to the actions of private individuals.

However, insulation from the exclusionary rule disappears when private individuals act jointly with the police, at the direction or request of the authorities, or as agents of the police. Suppression will also be ordered when police officers sit idly by and do nothing when they have knowledge that a private citizen is violating or about to violate the private rights of another, but mere consultation between victim and police does not create a joint operation.

Of course, police officers are private citizens too and, when they’re off-duty, they have only the authority of citizens to arrest and search. Actions by off-duty police officers as purely private citizens don’t fall within the exclusionary rule.