BONN, NOV. 3 -- Two West German policemen were shot and killed last night when they and their colleagues sought to break up a march called to protest a long-completed expansion of the Frankfurt airport.

It was the worst outbreak of violence during a political demonstration -- and the first time a policeman was killed during a protest -- since this country was founded in 1949.

The shootings were particularly alarming because they could mark the beginning of an increase in the use of violence by a group of anarchist militants known as "the Autonomes" who regularly fight with police but had not previously killed anyone, government officials and private experts said.

It was suspected that the shots were fired by someone in "Autonome circles," security sources said. Many of the demonstrators wore masks, as such militants frequently do to conceal their identities.

Interior Minister Friedrich Zimmermann called the shootings "a new phase and new dimension of violence."

Six other policemen were hospitalized, including two who were hit by gunfire, when about 200 police clashed with between 150 and 200 demonstrators outside the airport, according to police and West German news media reports.

The demonstration was called to mark the sixth anniversary of a police sweep that dismantled a "hut village" erected by protesters seeking to block construction of a third runway at the airport.

The protesters said at that time that trees should not be cut down to clear space for the runway, and that it would increase noise pollution. The runway was inaugurated in April 1984.

Police used water cannons to break up the marchers, who did not have a permit. The protesters tossed gasoline bombs and rocks, and fired tracer bullets as well as live ammunition.

Police detained an unidentified man today after a search of his apartment yielded a 9-mm pistol that fired the bullet that killed one of the two policemen, Alexander Prechtel, spokesman for the federal prosecutor's office, said tonight.

The pistol had been stolen from a policeman during an antinuclear-power demonstration in Hanau, near Frankfurt, on Nov. 8, 1986, the spokesman said. The detained man had been under investigation for sabotaging electric lines that carry power from nuclear plants, he said.

Tests showed that the pistol fired the shot that killed Klaus Eichoefer, 43, a police squadron leader, Prechtel said. It was not certain that it fired the bullets that killed policeman Thorsten Schwalm, 23, and wounded the two others, he said.

The incident was expected to lead to increased pressure from conservatives for the government to adopt controversial proposals to toughen penalties against the wearing of masks at political demonstrations.

The Autonomes, who are particularly active in Frankfurt, Berlin and Hamburg, frequently conceal their identities with ski masks, motorcycle helmets or kerchiefs. Chancellor Helmut Kohl's center-right coalition is divided over whether to strengthen the antidemonstration laws.

"The crimes in Frankfurt make it perfectly clear to all that violent demonstrators are using political disputes about state projects as an excuse to hit the state as such," Kohl's chief spokesman, Friedhelm Ost, said. "The state will not yield to violence."

"Normally the Autonome groups do not kill people, so this is something new," Gerd Langguth, a Bonn University political science professor who has written a book about West German protest movements, said in a telephone interview.

Langguth said that the Autonomes number between 2,000 and 3,000, and have battled with police at protests against nuclear power and against eviction of urban squatters. He cautioned that it was too early to say whether they are shifting to more violent tactics.

"Only one, two or three persons at a demonstration can decide that they want to shoot down policemen," Langguth said. "I'm convinced that not everybody in the Autonome groups are in favor of killing people."

Tilmann Chladek, a Bonn-based editor who has written about terrorism, suggested that hard-liners in the Autonome movement may have staged the shootings to galvanize the movement.

"There apparently is a discussion going on {in the movement} over whether the strategy of these violent demonstrations is helping," Chladek said in a telephone interview. "Perhaps somebody wanted to force the issue."

The prosecutor's office said someone in the crowd called on the police to stop advancing during the demonstration. Shortly afterward, the office said, a shout was heard: "Sharpshooters, fire!"

A local government spokesman said that the police were withdrawing at the moment when the policemen were shot and killed.