citizen's arrest

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An arrest made by a person other than a law enforcement officer for a crime committed in the presence of that person.
• noun An arrest made by a person who is not a law enforcement officer but who makes the arrest under the authorization of a law enforcement officer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law The right of a citizen to detain a criminal until the police can be summoned.