Passengers were taken out of the five terminals during the peak morning period and had to wait for two hours before being allowed back in for re-screening procedures.
"This (the evacuations) follows LAX's security procedures when it is learned that people have entered the secured areas without proper security screening," the airport said in a statement.
Stringent security measures have been put in place at airports around the nation since the Sept. 11 attacks on New York and Washington.
It was not known how many passengers may have boarded planes without proper screening before the problem was detected.
Authorities discovered at 6:30 a.m. that a metal detector was not working at Terminal 4, said Federal Aviation Administration spokesman Jerry Snyder.
"One of the metal detectors was found unplugged," Snyder said. "We do not know if it was malicious or not."
The five terminals, on the airport's south side, were emptied as a precaution because travelers could wander between the buildings once they cleared security, airport officials said.
Passengers jammed the sidewalks outside the terminals and began the re-screening at about 8:20 a.m., LAX spokesman Harold Johnson said.
About 325 departing flights from several major airlines, including American, Delta, United and Continental were delayed, he said.
Airport officials were unable to say how long flights were being delayed but said passengers were being advised to show up as normal, two hours before takeoff for check-in and screening procedures.
American Airlines spokesman John Hopkins said passengers should expect flight delays of up to several hours.
It was one of several cases of airport problems involving metal detectors reported this week.
On Tuesday, about 500 airline passengers in Buffalo (N.Y.) Niagara International Airport had to be re-screened after a metal detector was found partially unplugged. Ten flights were delayed.
On Monday, a terminal at Boston's Logan International Airport was evacuated and more than 1,200 passengers re-screened after officials discovered a detector hadn't been working for up to five minutes.
And not long after the Winter Olympics wrapped up late Sunday, flights at Salt Lake City's airport were held up because a metal detector was found not to have been working and some passengers required re-screening.