BONN, West Germany (AP) _ The government today condemned the assassination of a top Foreign Ministry official and called on citizens to help police catch the killers.

A masked assassin shot Gerold von Braunmuehl, 51, a close adviser to Foreign Minister Hans Dietrich Genscher, after he got out of a taxi in front of his Bonn home Friday night. The gunman escaped in a car driven by an accomplice.

Braunmuehl was director of the ministry's Political Department.

The Federal Prosecutor's Office today said the West German terrorist Red Army Faction claimed responsibility for the slaying in a letter found at the scene.

Prosecutor's spokesman Alexander Prechtel said Bonn authorities' initial report of a responsibility claim by a Revolutionary Front of Western Europe was incorrect. The letter urged the creation of such a revolutionary front, he said.

Prechtel said in a telephone interview from Karlsruhe that federal investigators determined the Red Army Faction claim was ''authentic'' but had no firm clues to the identities or whereabouts of the killers.

In Bonn, chief government spokesman Friedhelm Ost issued a statement denouncing the assassination as ''perfidious.''

''This cowardly murder arouses disgust and horror among all democrats,'' Ost said.

''The federal government, together with competent (security) officials will undertake everything possible to apprehend the murderers, and we call upon citizens for their help and cooperation in that regard,'' he said.

Braunmuehl, married and the father of three children, previously served at the West German embassies in Washington, Moscow and New Delhi in a diplomatic career starting in 1966, according to ministry spokesman Klaus-Peter Gottwald.

Braunmuehl left his office in a taxi at about 9:15 p.m. Friday and was shot minutes later after he got out in front of his home in the capital city's Ippendorf district.

The claim letter was signed ''Commando Ingrid Schubert - Red Army Faction'' and was stamped by the group's symbol, a five-cornered star with a machine gun superimposed, according to Prechtel.

Ms. Schubert, who hanged herself in prison nine years ago, was one of the original members of the Red Army Faction, a radical leftist terrorist organization blamed for a string of recent attacks.

The taxi driver, who had gotten out of the car with Braunmuehl to get the official's briefcase from the trunk, was not injured in the attack.

Braunmuehl was one of the most important and respected officials of the Foreign Ministry, according to ministry spokesman Juergen Chrobog.

The Political Department he headed handles basic foreign policy matters such as European unity and relations with the United States, the Soviet Union and the United Nations.

Prechtel said it was the first terrorist assassination of a ranking federal government official in Bonn in the country's 37-year history, and the first nationwide since Chief Federal Prosecutor Siegfried Buback was slain by the Red Army Faction in 1977.

Braunmuehl had been Genscher's personal office director until 1980 when he was reassigned to East European section, said Klaus-Peter Gottwald, also a Foreign Ministry spokesman. He had been in his current job for about a year.

Genscher, who went to the scene of the crime and also spoke with Braunmuehl's wife, ''learned of this new, brutal attack of political terrorism with consternation,'' Chrobog said.

Police said the office of chief federal prosecutor Kurt Rebmann had begun an investigation.

The Red Army Faction, which grew out of the Baader-Meinhof terrorist organization active in the late 1960s and 1970s, has been blamed by police for attacks including the July 9 assassination of Siemens company executive Karl Heinz Beckhurts and bomb blasts at several government buildings in August and September.

Ms. Schubert was arrested in 1970 and sentenced to six years in jail for attempted murder. Several bank robberies were later traced to her, and she was sentenced to another 13 years.

She hanged herself in her prison cell on Nov. 12, 1977 in Munich, four weeks after three other members of the group, Andreas Baader, Gudrun Ensslin and Jan-Carl Raspe, committed suicide in their cells in Stuttgart's Stammheim prison.

Another convicted terrorist, Ulrike Meinhof, had been found dead in prison in May 1976, in circumstances that were never made clear. Red Army Faction sympathizers claim she was murdered.

Baader and Ms. Meinhof founded the group, which originally was known by their names and later became the Red Army Faction. It has attacked American and West German military, government, business and industrial leaders and installations since the late 1960s.