Witnesses said Israeli helicopters fired three missiles at the wheelchair-bound Yassin and two bodyguards as they left the mosque, killing them instantly.

A total of four people were killed and 12 were wounded in the attack, witnesses said.

Yussef Haddad, 35, a taxi driver, said he saw the missiles hit 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. Since September 2000, 474 people - the majority of them Israelis - have been killed in 112 Palestinian suicide bombings.

Sharon's government has gone after militant leaders using Israeli helicopter gunships in a controversial policy that has resulted in a number of civilian casualties in addition to the deaths of senior figures in Hamas and other groups.

Yassin was viewed as an inspirational figure by his followers in the Gaza Strip and West Bank. His death could spur violent protests not only in the Palestinian areas but in the wider Arab and Islamic world, where he was well-regarded as a symbol of the Palestinian battle for independence.

Thousands of angry Palestinians gathered minutes after the attack, calling for revenge against Israel.

In announcing Yassin's death, Hamas said, "(Israeli Prime Minister Ariel) Sharon has opened the gates of hell and nothing will stop us from cutting off his head."

Ambulances and fire trucks raced to the scene with sirens wailing, and rescue workers gathered up the shattered bodies.

Outside the morgue at Shifa Hospital in Gaza City, Hamas official Ismail Haniyeh, a close associate of Yassin, had tears in his eyes as he confirmed Yassin's death and pledged revenge.

"This is the moment Sheik Yassin dreamed about," Haniyeh said. "Sheik Yassin lived and died and offered his life to Palestine. Sheik Yassin was a hero and a fighter and the leader of a nation, and (he) is in heaven now."

Yassin founded Hamas at the start of the first Palestinian intefadeh, or uprising, in 1987. It is an offshoot of the Islamic fundamentalist Muslim Brotherhood, which is based in Egypt.

The Bush administration, as well as Israel, lists Hamas as a terrorist group.

Yassin 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. Israel, which blamed him for inspiring Hamas bombers and attackers who killed hundreds of Israelis, tried unsuccessfully to kill him in a September missile strike that injured 16 people.