Hundreds of police, backed by water cannon and tear gas, charged into a public square for a second day Monday and scattered thousands of workers on the 20th anniversary of the suicide of a dissident student.

The police apparently attacked the workers when a group of them tried for a second time to place flowers at the site in Wenceslas Square where dissident Jan Palach set himself on fire in 1969 to protest the Soviet-led invasion of Czechoslovakia.Dazed and surprised workers scattered as police paddy wagons and water cannon roared into the square, bringing hundreds of riot police. A police officer speaking through a bullhorn ordered that the square be cleared, and when some refused to leave, the officers fired tear gas and charged.

There were no immediate reports of injuries, but a human rights activist said it was believed several arrests were made.

The incident occurred a day after riot police used tear gas and water cannon to disperse thousands of demonstrators who gathered in Wenceslas Square to mark Palach's death.

Official Polish Television said in its evening news program that more than 90 people were arrested in Sunday's demonstration, and activist sources said about 20 organizers of the memorial service were detained before the ceremony began.

The attack by police Monday came when 16 people who had been briefly detained after Sunday's demonstration tried for a second time to place flowers in the square.