Sheik Ahmed Yassin, founder of the radical Islamic group Hamas, is seen during a 1997 interview at his home in Gaza City. (Jacqueline Larma/AP File)
Gaza City, Gaza Strip — Israel assassinated Hamas founder Sheik Ahmed Yassin in a helicopter missile strike outside a Gaza City mosque on Monday, prompting threats of unprecedented revenge by Palestinian militants and leaving the already torn peace process in tatters.
Sheik Yassin was the most prominent Palestinian leader killed by Israel in more than three years of fighting, and his assassination was seen as a major escalation both in the Middle East and elsewhere.
More than 200,000 Palestinians, some carrying billowing green Hamas flags, flooded the streets for the funeral procession, the largest gathering in Gaza City in recent memory. Thousands also took to the streets in the West Bank.
Mourners jostled to touch his flag-draped coffin. Two Israeli helicopters flew above, and the sky was blackened from the smoke of tires set ablaze in the streets by protesting Palestinians.
At the cemetery, Sheik Yassin's body was carried between two rows of 200 extremists armed with anti-tank missiles and machine-guns.
"Words cannot describe the emotion of anger and hate inside our hearts," said Hamas official Ismail Haniyeh, a close associate of Sheik Yassin.
Israeli Prime Minister Ariel Sharon called Sheik Yassin the "mastermind of Palestinian terror" and a "mass murderer who is among Israel's greatest enemies."
Mr. Sharon said Israel will press ahead with its war on terror, signalling more "targeted attacks" and military raids. "The war against terror has not ended and will continue day after day, everywhere," he said.
U.S. National Security Adviser Condolezza Rice said Washington had "no advance warning" of the attack.
When asked about U.S. reaction to the attack during an interview on NBC television, Ms. Rice replied: "Let's remember that Hamas is a terrorist organization and that Sheik Yassin has himself, personally, we believe, been involved in terrorist planning."
State Department spokesman Lou Fintor said U.S. officials were in touch with Israeli and Palestinian authorities. "The United States urges all sides to remain calm and exercise restraint," he said.
The assassination was widely condemned in the Arab world and by some European countries. Egypt cancelled a trip by legislators and other dignitaries to Israel, which had been planned for later this month to mark the 25th anniversary of the signing of a peace treaty between the two countries.
British Foreign Secretary Jack Straw said the assassination "is unacceptable, it is unjustified and it is very unlikely to achieve its objectives."
"All of us understand Israel's need to defend itself against terrorism which affects it — within international law," said Mr. Straw, who was attending a meeting of European Union foreign ministers in Brussels. But Israel was not entitled to carry out "this kind of unlawful killing, which we condemn."
Israeli helicopters fired three missile as Sheik Yassin, his bodyguards and dozens of others left a mosque in Gaza City at daybreak Monday.
Sheik Yassin, a quadriplegic, and seven others were killed, including several bodyguards. Seventeen people were wounded.
Only a charred metal seat and a twisted wheel were left of his wheelchair, and a blood-soaked brown shoe lay in the street.
"Two or three people were lying next to him on the ground. One was legless," said taxi driver Yousef Haddad, who had rushed out of a nearby grocery when the missiles shook the Sabra neighbourhood.
Fearing reprisal attacks, Israel sealed off the West Bank and Gaza and confined many West Bank Palestinians to their communities. The Rafah border crossing between Gaza and Egypt was also closed. Troops reinforcements were sent to Gaza, and security forces in Israel were placed on high alert.
Three more Palestinians were killed in Gaza later Monday in clashes with Israeli troops, and another died while handling explosives.
Mr. Sharon, a former army general, was updated throughout the operation.
"The Israeli air force this morning killed the mastermind of all evil, Ahmed Yassin, who was a preacher of death," said army spokeswoman Brigadier-General Ruth Yaron.
The assassination was seen as an enormous gamble by Mr. Sharon, who is trying to score a decisive victory against Hamas ahead of a possible Israeli withdrawal from Gaza, but risks triggering a dramatic escalation in bloodshed that could turn the public's mood in Israel against him.
The Palestinian Authority said in a statement that "Israel has exceeded all red lines with this cheap and dirty crime." It declared a three-day mourning period.
Flags at Yasser Arafat's headquarters in the West Bank town of Ramallah were lowered to half-mast, and the Palestinian cabinet held an emergency session. Sheik Yassin was Arafat's biggest political rival, but Mr. Arafat was always careful not to confront the Hamas leader openly.
Cabinet ministers stood as Mr. Arafat recited a Muslim prayer for the dead. The Palestinian leader, referring to Sheik Yassin, then added: "May you join the martyrs and the prophets. To heaven, you martyr."
Earlier Monday, about 2,000 demonstrators gathered at Mr. Arafat's headquarters, calling for revenge. He remained inside.
An Israeli security official said that there are no immediate plans to kill the Palestinian president and that Hamas is the focus of Israel's current offensive.
Tens of thousands of Palestinians marched in the West Bank in protest, including about 15,000 people in Nablus. "This morning, dozens came to us volunteering to be suicide bombers and we will send them at the right time," one masked man said at the rally.
Thousands more demonstrated in the town of Jenin. In the West Bank town of Hebron, Palestinians threw stones at Israeli soldiers, who fired tear gas.
West Bank schools were closed, and a one-day commercial strike was declared. In the Israeli prison camp of Ketziot, hundreds of Palestinian prisoners rioted briefly, setting tents on fire and throwing stones at soldiers.
Hamas threatened a harsh response.
"Yassin is a man in a nation, and a nation in a man. And the retaliation of this nation will be of the size of this man," said Abdel Aziz Rantisi, a prominent Hamas leader in Gaza who escaped an Israeli assassination attempt last June.
For the first time, Hamas also threatened the United States, saying that only Washington's strong backing of Israel made the assassination possible.
Ms. Rice and Israeli Foreign Minister Silvan Shalom said Monday that Israel gave no advance warning of the attack.
In the past, Hamas leaders have insisted that their struggle is against Israel and that they would not get involved in causes by militant Muslims in other parts of the world. Monday's statement suggested that Hamas might seek outside help in carrying out revenge attacks.
Rival extremist groups also promised revenge.
Israel had previously tried to kill Sheik Yassin in September when a plane dropped a bomb on a building where he and other Hamas leaders were meeting. He escaped with a small wound to his hand.
