About 200 people gathered in Prague's Wenceslas Square Saturday on the 20th anniversary of the Warsaw Pact invasion that crushed reform in 1968.

They discussed the need for more freedom in Czechoslovakia as uniformed and plainclothes police stood near the gathering and checked identity documents.Participants had responded to leaflets distributed to mailboxes by a group called the "Critical Citizens."

Earlier, police detained the three spokesmen of the human rights group Charter 77 as they walked to the Soviet Embassy with a statement demanding that the Kremlin tell the truth about the invasion by Soviet and Warsaw Pact tanks and troops on Aug. 20-21, 1968.

The invasion crushed the "Prague Spring" of reform pioneered by then-Communist Party leader Alexander Dubcek, who was ousted in April 1969 and expelled from the Communist Party in 1970.

Dubcek said in Bratislava this week he planned to spend the anniversary quietly at his country cottage.

Despite pouring rain, about 200 people converged on Wenceslas Square, the scene of fierce protests against the 1968 invasion.

The gathering reflected increased activism by a handful of independent groups formed in Czechoslovakia since Mikhail S. Gorbachev began implementing his reforms in the Soviet Union over the past three years.

A group of about 100 sang the Czech and Slovak national anthems, then discussed what could be done to change the country. Some called for more religious freedom, democratic elections and constitutional reform.

Roman Catholic believers in Czechoslovakia have grown more active in recent months. About a half million signed a petition to communist authorities this year demanding more bishops for vacant sees, more priests and an end to alleged harassment of believers.

A young man who said he was from Slovakia appeared to lead the hourlong discussion, in which there was no specific mention of the 1968 invasion. Western diplomats said those who had initiated the discussion were not known activists.

A group called "Czech Children" also distributed leaflets calling for a discussion on Wenceslas Square Sunday.

Charter 77 spokesmen Stanislav Devaty, Milos Hajek and Bohumir Janat had announced plans to lay flowers on the square, at the Central Committee building and at state radio to commemorate countrymen who were killed or injured in violence at those sites after the invasion.

But an unidentified man at the square told the crowd: "Those with the stronger experience are not here. They are in jail. Now the spokesmen of Charter 77 are in the police station."

It was presumed the three spokesmen would be detained for 48 hours and then released. Czechoslovak law allows detention for 48 hours without charge.

Police warned other leading group members before the weekend they would be detained, and left town for the anniversary.

Police presence was heavy in the capital Saturday, but few citizens were expected to make any public display of emotion over the anniversary.

Commentaries in the state-run media continued to reflect the official view that the invasion was a painful but necessary move because Dubcek and other leaders allowed reform to veer out of control, threatening Czechoslovakia's ties to the Warsaw Pact.