GAZA CITY, Gaza Strip (AP) _ Israeli helicopters fired missiles at Hamas leader Sheik Ahmed Yassin as he left a mosque near his house at daybreak Monday, residents said, and Hamas officials and witnesses said he was killed.

Witnesses said Israeli helicopters fired three missiles at Yassin and two bodyguards as they left the mosque, killing them instantly. Hamas officials confirmed that he had been killed. Four people were killed and 12 wounded in the attack, witnesses said.

Yussef Haddad, 35, a taxi driver, said he saw the missiles hit and kill Yassin and the bodyguards. ``Their bodies were shattered,'' he said.

Yassin was by far the most senior Palestinian militant killed in more than three years of Israeli-Palestinian fighting. Thousands of angry Palestinians gathered around his minutes after the attack, calling for revenge against Israel.

Announcing Yassin's death, the Hamas leadership said, ``Sharon has opened the gates of hell. and nothing will stop us from cutting off his head,'' referring to Israeli Prime Minister Ariel Sharon.''

Ambulances and fire trucks raced to the scene. sirens wailing, and rescue workers were gathering up parts of the shattered bodies.

Yassin, a quadriplegic, founded Hamas in 1987. He was held in Israeli prisons for several years before being released in 1994.

Yassin lived in a modest house in the rundown Sabra neighborhood in Gaza City. Though he was limited in his movements, and Israel blamed him for inspiring Hamas bombers and attackers who killed hundreds of Israelis, Israeli governments had until now refrained from targeting him, fearing a firestorm of revenge attacks.