A loud explosion, believed caused by a bomb, shattered windows at the Cuban diplomatic mission on 16th Street NW just before midnight last night, authorities reported. There were no reports of injuries.

Authorities said the blast occurred in the rear of the elegant sandstone building at 2630 16th St. that once housed the Cuban Embassy and now is occupied by the Cuban Interests Section.

More than an hour after the blast a man called the Associated Press and said an anti-Castro Cuban group called Omega 7 claimed responsibility for the explosion.

"We demand the withdrawal of Cuban troops on the African continent where thousands of young Cubans die daily, forced by dictator Fidel Castro to fight an unjust war," the man, who spoke with a slight Spanish accent, said in the 1:15 a.m. call.

Omega 7 and a larger Cuban nationalist groups of which it is part have been linked to several bombings in the United States in recent years.

Authorities said a lookout was issued for two men of undetermined age reportedly seen running from the area of the mission atop Meridian Hill about the time the blast was heard. One of the men reportedly carried a handgun. The two were wearing raincoats, police reported.

Police said they believed the bomb, which was detonated about 11:50 p.m., was tossed into the back yard of the three-story mission by the men, who fled immediately.

"There was an explosion and it shook my car," said one policeman who had been on patrol nearby. "I thought someone was after me."

Heard for blocks around, the blast broke at least seven windows in the Cuban mission, and left the yard litered with shards of glass. Windows in other nearby buildings were also broken, authorities said and a window air conditioner in the Cuban building was pushed backward into the building.

Details about the explosive device were not immediately available, and one police source said it apparently had disintegrated in the explosion. Investigators from the FBI and D.C. police were on the scene last night along with members of the uniformed division of the Secret Service, which protects foreign diplomatic missions here. Top D.C. police officials were alerted by radio. Fire trucks were sent but were not needed.

Shortly after the explosion a man who answered the telephone at the mission said no information could be provided immediately, but when asked whether the blast had been caused by a bomb, replied in Spanish "parece ser ,"-"it seems to be."

Later, a switchboard operator said she was instructed to say officials would have no comment.

The Cuban Interests Section was opened here on Sept. 1, 1977, in a move that ended almost 17 years of diplomatic separation between the United States and Cuba. A United States Interests Section was opened in Havana at the same time.

While serving as the Cuban Embassy in the days before Castro, the building that now houses the mission was home to glamorous parties and lavish entertaining. In 1963, two years after it was closed, shrubbery in front was damaged by molotov cocktails allegedly hurled by Cuban veterans of the Bay of Pigs fighting.

At the time the building was being cared for by Czechoslovakia, which had been asked to occupy it in the Cubans' absence. CAPTION: Picture, Bomb disposal truck enters front gate this morning at 16th Street building that a houses Cuban mission. By Linda Wheeler-The Washington Post