As thousands of workers staged anti-Communist demonstrations across Poland yesterday, about 100 people gathered in Lafayette Square here to express support for Solidarity, the independent trade union that was banned by the Polish government when it imposed martial law in December 1981.

In a message read to the gathering, President Reagan denounced "the continuing wave of suppression" by the Polish government, which he said was being "carried out under intense Soviet pressure."

But the main speaker at the rally, former U.S. ambassador to Poland Richard Davies criticized the Reagan administration for "collaborating with the oppressors of the Polish people."

Davies, who served in Warsaw from 1973 to 1978, said Reagan's April 22 decision to resume negotiations for a long-term grain agreement with the Russians reduced pressure on Soviet leaders. He said the administration had also shown an "indulgent attitude toward the (Polish) junta" by allowing American and West European bankers to give the Warsaw government more time to pay off debts.

In his statement, read by State Department aide Lawrence Roeder, Reagan repeated his pledge of last December that the United States would help Poland solve its economic problems if the Communist government "introduces meaningful liberalization measures." But he warned: "Such steps cannot be merely cosmetic."

"What we have witnessed is a genuine struggle of workers in a so-called workers' state for basic human and economic rights," the president declared. "We salute the courage of the Poles who daily face intimidation and harassment. Their heroic resistance to their government's attempts to suppress Solidarity serves as an inspiring display of the indomitable desire for freedom. . . ."

The rally, which featured "Solidarnosc" banners in Polish and a poster of labor leader Lech Walesa, was organized by Human Rights International, an offshoot of Mensa, the high IQ society.