GAZA CITY, Gaza Strip ? Sheik Ahmed Yassin, the founder and leader of the Hamas militant group, was killed by missiles fired from Israeli helicopters as he left a mosque at daybreak today, witnesses said.

Hamas confirmed the death in an announcement broadcast over mosque loudspeakers and vowed revenge against Israeli Prime Minister Ariel Sharon.

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

Four people were killed and 12 were wounded in the attack, witnesses said.

Yassin, a quadriplegic, was by far the most senior Palestinian militant killed in more than three years of Israeli-Palestinian fighting.

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.”

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

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.

Past Israeli governments were reluctant to target Yassin, fearing a firestorm of revenge attacks.