PRAGUE, Czechoslovakia (AP) _ Police forcibly broke up a peace demonstration Saturday in Wenceslas Square and detained 47 people, the state news agency CTK said.

Protesters from The Independent Peace Association and Czech Children groups gathered Saturday evening near the statue of St. Wenceslas, where demonstrators last month tried to gather the day before the 11th anniversary of the Soviet-led invasion.

On Aug. 21, 1968, Warsaw Pact troops and tanks moved into Czechoslovakia to quash reforms being pioneered by then-Communist leader Alexander Dubcek.

Police filmed activists as plainclothes and uniformed police waded into the crowd, checking identification cards of the participants.

''Why are you here?'' one policeman asked. ''You are obstructing traffic.''

When the police took away Herman Chromy, a member of the independent Charter 77 human rights group, the crowd whistled and yelled, ''Shame 3/8 Shame 3/8''

Four leading members of the Peace Association were detained earlier Saturday, with three placed under house arrest, including Tomas Dvorak and Hana Marvanova, activists said.

Police forced the detainees onto a bus by twisting their arms behind their backs and pushing them, activists said.

In a two-paragraph dispatch, the news agency CTK described the meeting as an ''attempted infringement of public order.'' It said about 100 people exhibiting ''eccentric behavior ... attracted the attention of some 400 passerbys and foreign tourists.''

CTK said 47 people were taken to police stations for ''further proceedings.'' It said police ''had to intervene'' because people disregarded repeated calls to disperse.

Plainclothes police also checked the identification of two Western journalists. They grabbed an Associated Press staffer by the arm and threatened to expel her from the country.

The police then withdrew the threat on condition the two journalists stop taking notes and leave the area.

Later three plainclothes policemen surrounded a Reuters correspondent, holding him by both arms and twisting his notebook out of his hand.

To clear the area, about 70 uniformed police wielding clubs swept down into the square, repeatedly urging people over a loudspeaker to ''keep walking.''