Police fired water cannons and hurled tear gas Friday to disperse about 5,000 people who defied a ban on a rally marking the 70th anniversary of Czechoslovakia's independence.

Helmeted police wielding batons swarmed through Prague's Wenceslas Square to break up the crowd, which chanted "Gestapo!" as the police charged.Earlier, police arrested about 70 dissidents throughout the country in one of the biggest raids on independent activists in years.

At least 20 arrests were made in Wenceslas Square, and some of those detained were beaten by police or had their arms twisted by plainclothesmen as they were taken to buses and driven away.

Authorities banned the independent rally, called a week ago by the Charter 77 human rights group and four other independent organization.

But people came to Wenceslas Square at the rallying hour of 3 p.m.

Hundreds of people in the center of the square began singing the Czechoslovak national anthem when police gave the first order to disperse, and thousands of onlookers joined in.

Chants of "Freedom!" "Long Live Freedom!" and "Masaryk!" rang out, despite police warnings to disperse.

Thomas Garrigue Masaryk was the founder and first president of the democratic republic of Czechoslovakia, founded Oct. 28, 1918.

Police moved down the square, hitting with batons onlookers who refused to move.

Communist authorities held an officially sanctioned independence rally Thursday and repeatedly warned people not to attend the Friday ceremony.

Also Thursday, President Gustav Husak declared a limited amnesty that could free a handful of political prisoners, said Anna Sabatova, a member of the independent Committee for the Unjustly Prosecuted.

The country's general prosecutor, Jan Pjescak, was quoted by the Communist party daily Rude Pravo as saying the amnesty was the biggest in years. About 7,000 prisoners are expected to go free and 160,000 other people will have charges dropped, Pjescak said.

The action seemed intended to pardon first-time offenders convicted of many kinds of crime and many people who have left the country illegally.