PRAGUE, Czechoslovakia (AP) _ The government put more police in the streets Friday on the eve of the 20th anniversary of the Soviet-led invasion that crushed Alexander Dubcek's liberal ''Prague spring'' of 1968.

Official media concentrated on criticizing the former Communist Party chief and other sponsors of reforms that resembled those now pursued by Mikhail S. Gorbachev but did not please the Kremlin of the time.

Police ranks also were reinforced in the Slovak capital of Bratislava, where Dubcek lives. The disgraced former leader, now 67, said he would spend the anniversary quietly with his grandchildren.

Dubcek refused an interview earlier this week when asked by Western reporters. ''They will be angry with me,'' he said of the authorities who gave him a minor forestry job in Bratislava in 1970 and have kept him under close watch.

Whatever the police presence, Czechoslovakia's dissident movement was at work.

Leaflets stuffed in some city mailboxes urged citizens to gather in Wenceslas Square, scene of protests 1968, on Saturday night for a discussion of the invasion.

Spokesmen for Charter 77, the human rights group, planned to deliver a statement to the Soviet Embassy at noon Saturday demanding a full account of the invasion from the Kremlin. They also planned to lay flowers at the embassy in memory of Czechoslovaks killed in the invasion and ensuing street violence.

Most of the 15.5 million Czechoslovaks fell silent after the invasion of Aug. 20-21, 1968, and few are expected to show their feelings openly this weekend about the events of a generation ago.

''This anniversary is not being celebrated,'' government spokesman Miroslav Pavel said. ''We see no reasons to celebrate.''

Authorities expelled about 20 West Europeans, most of them Italians, who hung a banner across Wenceslas Square on Thursday demanding freedom in Czechoslovakia and withdrawal of Soviet troops.

On Thursday and Friday, state television broadcast a 90-minute film showing Dubcek and other former leaders expelled from the party with him in a purge of half a million members overseen by Milos Jakes, the current party chief.

It reflected the line taken by state-run media in recent weeks: that changes were needed in 1968, but Dubcek and his allies were weak leaders who permitted events get out of control.

Several academics and district party leaders were shown recalling the events of 1968 and blaming Dubcek and other leaders, who enjoyed enormous popularity, for precipitating the invasion.

''We hoped that a process of democratization would take place that would open room for positive changes,'' writer Miroslav Hlatky said.

''Most people lived in the euphoria of those days and expected just a socialist (communist) system. But all of a sudden the process of democratization was strangled (by anti-communist forces) and the human face of socialism they were wearing changed into a hard and evil mask.''

Hlatky's allusion was to Dubcek's stated wish to create ''socialism with a human face.''

Gorbachev's adoption of the phrase and his own zeal for reform have prompted speculation Moscow would change its stance on the invasion, whose official reason was to stop a counterrevolution that might take Czechoslovakia out of the Warsaw Pact.

Although Soviet historians have made clear there is more official sympathy for Dubcek than before, Kremlin spokesman Gennady I. Gerasimov reiterated this week that the Soviet party ''respects'' the official Czechoslovak judgment that the invasion was necessary.

On Friday, the Soviet news agency Tass said the Soviet Union's intervention saved the country from irresponsible leadership and possible brutual repression of communists.

''Anti-socialist forces, taking advantage of political irresponsibilty and opportunism of a part of the leadership of the Communist Party of Czechoslovakia of that time, made an attempt ... to tear Czechoslovakia away from the socialist community,'' the Tass commentary said. ''The friends and allies of socialist Czechoslovakia could not remain indifferent.''

Czechoslovak media repeated that official view for the past month but Milan Matous, a party theorist well-known for justifying the invasion, conceded in two articles that a ''deeper and more critical'' evaluation might be made.

Pavel, the government spokesman, indicated to reporters Thursday a commission of Soviet and Czechoslovak historians might be formed. In an interview Friday with The Associated Press, however, he said that was ''just a private idea. It is not under discussion.''

Cautious economic reforms have been made since Jakes assumed the leadership last December, but political change seems distant.