A large section of ceiling came crashing down at Farragut North, Metro's third-busiest subway station, just as the evening rush hour began yesterday.

The 20-by-20-foot section fell on the mezzanine area at the station's southwest entrance at L Street and Connecticut Avenue. The station entrance is underneath the Washington Square complex of shops, restaurants and offices.

At 3:56 p.m., just after a man had walked through the passageway between the escalators and the fare gates, a loud noise erupted and the ceiling fell, said Linda Rhone, the station manager. "There was a loud boom and then just black clouds -- from the soot," she said. "I couldn't even see anything; it was all black."

No one was walking beneath the ceiling when it fell and no one was injured. Metro officials closed that entrance to the station, which has two other entrances -- one across Connecticut Avenue from the closed entrance, and one on K Street. Trains on the Red Line were not affected.

Farragut North is on the oldest stretch of the subway, which opened in 1976. A total of 50,404 passengers use the station on an average weekday, according to Metro. On a typical weekday, 12,774 passengers use the entrance where the ceiling fell.

The fallen section was part of a drop ceiling consisting of metal, ceiling tiles and insulation. About five light fixtures also came down in the crash.

Directly above the affected area is a Victoria's Secret shop. Salesclerk Kristin Lord said she didn't hear, feel or notice anything unusual when the ceiling collapsed in the station. "That is so strange," said Lord, who was ringing up lingerie at the cash register.

As rush hour commuters streamed into other parts of the station, the collapsed section lay in a dirty heap. Metro workers were trying to determine why the ceiling fell.

"It's really too early to know the exact cause," said Lisa Farbstein, a Metro spokeswoman. "It's likely as our system ages we're going to have to increase maintenance of our infrastructure, especially the older stations, many of which are on the Red Line, like this one."

Metro officials have said the transit system needs $1.5 billion in the next six years to buy rail cars and buses and to rehabilitate stations and equipment. So far, none of the local jurisdictions that fund Metro or the federal government has agreed to pay those costs.

Farbstein said Metro expected to have the entrance back in service this morning.