hey filled the broad lanes of West Berlin's main thoroughfare, forming a colorful and generally orderly phalanx that stretched more than 10 blocks, broadcasting a message that came to one main point: We demonstrating West Germans are afraid of America.

Some carried U.S. flags in which the stars had been replaced with tombstones. A few carried coffins, draped in U.S. flags. The banners they waved bore the renouncing slogans of West Germany's self-proclaimed and growing peace movement.

"No to the neutron bomb," they said, and "We won't let ourselves be protected to death." One sign in English put the problem as "to be or NATO be." Another warned visiting Secretary of State Alexander M. Haig Jr., "Don't play with us, Mr. Haig, we don't want to die."

Today's demonstration--by tens of thousands of mostly youthful opponents of the plan to station medium-range nuclear missiles in Western Europe and other aspects of U.S. foreign policy--was one of the largest marches by the movement this year.

It was taken as an embarrassment by a majority of West Berliners, who continue to pay homage to the United States for the security guarantee it provides this city--Germany's largest, located deep inside East German territory.

The demonstration was also severely criticized in its planning stages by senior Bonn officials, especially sensitive to the fact that among the main organizers were the youth groups of Chancellor Helmut Schmidt's Social Democratic Party and Foreign Minister Hans-Dietrich Genscher's Free Democratic Party.

But the party elders were unable to prevent their large and highly politicized youth groups from going ahead with the march. Some West German commentators said this resulted from a lack of any threat of punishment to back up pleas by Social Democratic Party Chairman Willy Brandt and others not to participate.

More than 50 groups took part in the demonstration, ranging from church clubs to communist cells. Police estimated the number of marchers at 30,000, while the organizers said there were 80,000.

In any case, there were more than enough to fill the huge Winterfeldtplatz, where the march stopped, not far from the Schoeneberger City Hall, where Haig had gone to sign the city's guest book.

Under the broken clouds of the warm day, the protesters heard a series of speakers strike the major themes of their campaign: that the Reagan administration was posing a threat to world peace by rearming, that Europe would be the victim of the new cold-war tensions between the superpowers and that the only way out was for Europe to resist NATO's modernization plans and stop the arms race.

All who spoke said the Berlin demonstration was not against America but against the policies of the Reagan administration. Dorothy Soelle, a theologian, talked about "the other America" that had raised its voice for peace in the Vietnam War years.

Erich Fried, a London-based author, criticized the burning of an American flag he had witnessed earlier in the day. U.S. flags should not be burned, he said, but rather symbolically washed clean.

Haig, while regarded by the Bonn government as a champion of West European interests in the Reagan administration, was the target of one motto offered by the protesters: "There is nothing more important than peace." This was a reference to Haig's remark before the Senate Foreign Relations Committee in January that "there are more important things than peace"--a statement he later explained had been meant to suggest that there are things Americans must remain willing to fight for.