BEIT HANOUN, Gaza Strip — With both legs badly bruised from a vicious beating, Shaher Abu Oda can get around only with a painful shuffle.

In the Gaza Strip town of Beit Hanoun, however, he is just one of many young men bearing limps, plaster casts and stitches — the black-and-blue aftermath of an unprecedented crackdown on dissent by Gaza's new rulers, the Islamist group Hamas.

Hamas officials snatched Mr. Abu Oda off the streets two weeks ago as he tried to find his younger brother Miqbil, himself badly beaten after club-wielding Hamas policemen broke up a wedding party.

The revelers' crime had been to sing a few songs associated with the Fatah party, the rival Palestinian faction that Hamas ousted from the Gaza Strip two months ago.

"They threw me in a room," said Mr. Abu Oda. "From 11:30 to 3:30 in the morning, they came in every 15 minutes and beat me with sticks, fists, kicks and a black leather crop."

As many as 50 people are thought to have been arrested in Gaza's Beit Hanoun district around the night of the wedding, and similar sweeps have taken place elsewhere in Gaza since then.

The detentions and beatings appear to mark the end of a relative honeymoon period for Hamas, which seized control of Gaza after five days of battle in June.