BRUSSELS, Belgium (AP) _ A bomb explosion damaged the offices of a major West Germanelectronics firm Sunday, 20 hours after a bomb set fire to the North Atlantic Assembly building.

No one was harmed in Sunday's blast at the AEG Telefunken building. Two people were slightly injured in the explosion Saturday.

Responsibility for the bombing at the downtown offices of the North Atlantic Assembly was claimed on behalf of the previously unknown Revolutionary Front for Proletarian Action in an anonymous phone call to Belgian radio.

No one immediately claimed responsibility for Sunday's blast, but the word ''FRAP'' - the Revolutionary Front's initials in French - was spray-painted in red on a wall in the garden of the AEG building. The word also means ''strike'' or ''hit'' in phonetic French. The same word was painted on an outside wall of the North Atlantic Assembly building.

Police said the pre-dawn explosion at AEG caused no injuries but blew out windows on several floors in the back of the modern, six-story building, and shattered all the windows in a glass corridor leading to a second building nearby.

In both cases, explosives were placed on outside window sills in the rear of the buildings, located several miles away from the NATO headquarters here.

The blasts were the ninth and tenth bomb attacks against NATO and NATO- related targets in Belgium in the past seven months. AEG, West Germany's third-largest electronics firm, is a contractor for the West German military.

AEG spokesman Karl-Heinz Rumpf said in Hanover, West Germany, that his firm's Brussels office is not involved in military contracts.

It oversees marketing of household appliances and electric components for energy systems, Rumpf said.

The subsidiary is in Brussels' southern suburb of Uccle, where many foreign firms are based.

Since Oct. 2, 1984, there has been a series of bomb attacks throughout Belgium against NATO installations and Western defense contractors.

The attacks, including six bomb explosions along NATO's emergency fuel pipeline system on Dec. 11, have caused much damage but no deaths or serious injuries.

An extreme leftwing group calling itself the Fighting Communist Cells has claimed responsibilitity for the previous explosions.

Officials believe the Fighting Communist Cells have links with West German and French terrorist groups.

Saturday's blast at the North Atlantic Assembly secretariat set off a fire in an archives room, but most damage was sustained by a restored 17th century house across a narrow alley in the back. Two people, including an Italian diplomat living in the house, were slightly injured by flying glass, officials said.

The 30-year-old North Atlantic Assembly is an organization of 184 members of parliament of the 16 NATO nations. It is not part of NATO political decision-making or military structure, but provides a link between the NATO and the legislatures in its member states.