Sheik Ahmed Yassin, the spiritual leader and founder of the militant Palestinian group Hamas, was killed early Monday by an Israeli missile that struck him as he left a mosque in Gaza City, his family and Hamas officials said. They said at least two bodyguards had been killed with him.

Sheik Yassin, a symbol to Palestinians of resistance to Israel and to Israelis of Palestinian terrorism, was by far the most significant Palestinian militant killed by Israel in more than three years of conflict.

Black smoke curled over Gaza City as Palestinians began burning tires in the streets and demonstrators chanted for revenge. Mosque loudspeakers blared a message across Gaza of mourning for Sheik Yassin in the name of Hamas and another militant group, Al Aksa Martyrs Brigades.

The Israeli military confirmed the killing, saying in a statement that the sheik was ''responsible for numerous murderous terror attacks, resulting in the deaths of many civilians, both Israeli and foreign.''