More than 300,000 persons protested U.S. defense policies here today in what was believed to be the largest rally in Germany since World War II as President Reagan and other leaders conferred at the NATO summit.

The number of demonstrators topped even those who gathered here last October to protest the U.S. arms buildup and followed appeals by the president in the British Parliament and the Bundestag here this week aimed at winning over those opposed to strengthening the NATO defense posture in Europe.

"The peace movement will not let itself be misled by empty disarmament slogans," declared Jo Leinen, spokesman for the rally's coordinating committee. "This is why no one in the peace movement can imagine Ronald Reagan marching at the head of a peace demonstration, as he said yesterday in the West German parliament."

Reagan's arms-reduction proposals, outlined in his speech yesterday, and his less stinging tone toward Moscow have been welcomed by officials here and by much of the West German media as a sign of a U.S. return to a partial detente policy.

But many of Europe's disarmament campaigners continue to doubt Reagan's gestures and such claims of understanding for their cause as the president expressed yesterday, viewing Reagan's moves suspiciously as little more than clever public relations ploys.

The enormous size and confident, relaxed spirit of the Bonn rally would suggest that Reagan's efforts have done little to stem the momentum of the grass-roots drive here against nuclear weapons that he helped spark early in his term.

The massive demonstration, held on a broad, grassy field across the Rhine River from the government complex where Reagan and 15 other leaders of the North Atlantic Treaty Organization met today, had the appearance of a grand summer festival. In warm, sunny weather, the youthful crowd streamed into the area by train and caravans of buses that clogged the highways leading to the city.

Although many of them ended up out of earshot of the speeches, it did not seem to matter. They had come mainly to show themselves, to lounge on the grass or cool their feet in the river, to picnic and to browse among the bookstands, health food stores and assorted stands selling symbols for a peace theme that has become good business as well as good politics.

Despite splits and dissension among leading disarmament groups over the aims of the movement, there was little of the meanness or violence that had been feared. The peaceful and generally orderly character of the rally reflected the experience young organizers have acquired staging such public demonstrations in recent months, and the interest those who came had in disarming their own critics.

Amid reports that the West German disarmament campaign was being managed increasingly by Communists and pushed into an anti-Western stance, the country's major left-wing groups rallied together in the last few days to proclaim a common purpose in the rally.

Initially, the two major organizers of last October's rally--Action For Reconciliation, a West Berlin-based group with ties to the Evangelical Lutheran Church, and the Greens, a national environmental party--had sought to avoid the demonstration, upset about what they feared would be its one-sided, anti-American tilt.

But leaders for both groups ended up endorsing the gathering, and the rally's final declaration included a general call for a worldwide stop of armaments and complete withdrawal of nuclear weapons from both Eastern and Western Europe.

Answering charges of anti-Americanism, spokesmen today claimed solidarity with the U.S. grass-roots campaign for a nuclear freeze.

The Rev. Robert Davidson, a Presbyterian clergyman from New York City, told the rally he brought greetings of solidarity from the nuclear freeze campaign in the United States.

Thousands in the crowd wore orange "Reagan, go home" stickers.

A grouping of Marxist organizations staged a separate march in the northern part of Bonn opposing "NATO imperialism." Police estimated that the Marxist protest drew about 20,000 persons.

While the protests generally remained very calm, about 2,000 rowdy demonstrators found their way past police to the area around the chancellor's offices. Marching up Bonn's main government avenue, they pulled down several NATO, U.S. and British flags and burned them before police pushed them back without serious incident.

The main reported casualty of the day was an unidentified person at the main rally who tried first to stab himself, then immolate himself. He was rushed to a hospital, where he was reported in grave condition.