Underground Solidarity leaders have described widespread demonstrations last week as a moral victory for their suspended trade union and warned that time is running out for a peaceful solution to Poland's problems.

In a statement that reached Western correspondents here today, the five members of Solidarity's provisional coordinating commission called on workers to observe a minute's silence at the end of this month in memory of demonstrators killed during clashes with police on Aug. 31. But they stopped short of announcing any further demonstrations.

The statement was the first public comment by Solidarity's underground leadership since the demonstrations, which marked the second anniversary of the agreement that legalized independent unions. The Communist authorities described the protests as a failure, ill-attended and unrepresentative.

Even government officials, however, admitted that the demonstrations amounted to the biggest display of opposition to martial law since the December military takeover. They were particularly strong in the southwest, where at least four people were killed as police fired into crowds.

According to official accounts, protests flared up in 34 of the 49 provinces, more than 4,000 demonstrators were arrested and 200 wounded, including 148 policemen. The Solidarity leaders said demonstrations were peaceful where the police did not attempt to break them up.

"In its response to Solidarity's appeal for social peace in July and for demonstrations in August, the public proved its discipline and unity," the statement said. "Such a nation cannot be ruled by violence. If the authorities do not understand this, if they do not start negotiations with Solidarity or with its leader, Lech Walesa, we may lose a chance for a peaceful solution of the conflict."

The relatively restrained language of the Solidarity statement suggested that the union's leaders wish to analyze the long-term effects of last week's demonstrations before deciding on further action.

[Polish Archbishop Jozef Glemp of Warsaw will arrive in Washington Oct. 14 to begin a two-week tour, according to officials at the U.S. Catholic Conference. He is to celebrate mass at the Shrine of the Immaculate Conception Oct. 15, then visit a dozen cities with high concentrations of Polish-Americans.]