The Soviet Union confirmed Thursday that police clashed a day earlier with pro-independence demonstrators in the Lithuanian capital of Vilnius and arrested about 20 people.

The Tass news agency said police stopped the unofficial demonstration marking the 49th anniversary of a Soviet-Nazi pact providing for the Soviet annexation of Lithuania during World War II."The city authorities had disallowed the meeting. The law enforcement agencies prevented it from being held," it said.

Dissident sources in Vilnius reached by telephone from Washington Wednesday said police armed with truncheons attacked a crowd of about 5,000 demonstrators and injured at least 20 people. The sources said between 20 and 30 people were seen being forced into police vans and driven away.

They said protestors fought back with scraps of wood from a construction site.