An explosion that police said was caused by a bomb blew out a front window late last night at the downtown Washington offices of Aeroflot, the Soviet airline.

The blast, heard for blocks around, came about 11:15 p.m. at the entrance to the offices at 16th and L Streets NW. It was at least the fourth explosion at the building in the last six years. The most recent came last Feb. 19, almost exactly a year ago.

No injuries were reported in last night's blast, which blew out a narrow five- or six-foot-high window adjacent to the front doors. The doors apparently were scratched or pitted by the explosion. The second letter "O" in the Cyrillic alphabet spelling of Aeroflot was dislodged from a sign on the facade of the building and fell to the sidewalk.

Police said last night they had no suspects and knew of no attempt to claim responsibility. Investigators working under floodlights were gathering evidence at the cordoned-off scene early today.

No arrests have been made in the previous incidents. The blast causing the heaviest damage occurred Sept. 7, 1977, and blew out windows in nearby buildings. A telephone call to a wire service office said that bomb was in protest of Soviet support of Cuba.

Last year's explosion, believed caused by a pipe bomb, came minutes before The Washington Post received a call from a person who said: "This is the Jewish Defense League." The caller, only partly audible, claimed the explosion was in memory of an anniversary connected with a Soviet woman being punished because she wanted to go to Israel.

Within a few minutes after last night's blast, police halted traffic nearby, fire trucks laid hose in the street, an ambulance pulled up and investigators from several agencies arrived. Onlookers also assembled. Some said they had heard the blast blocks away.

The offices are about 150 feet south of the Soviet Embassy on 16th Street. Aeroflot service here was suspended early last year by the president in retaliation for alleged Soviet involvement in the crackdown in Poland.