JERUSALEM -- Israel's assassination of Hamas spiritual leader Sheik Ahmed Yassin was a huge gamble.

It could either weaken Hamas and drive its leadership underground or galvanize Palestinian hatred toward Israel and end up strengthening the Islamic militant group.

New pressures may have pushed Israeli Prime Minister Ariel Sharon to act now.

Sharon has been trying to persuade reluctant hard-line members of his Likud Party to accept his plan to withdraw from Gaza. He has stepped up attacks there in recent weeks, trying to weaken the militant groups and dispel the image they are chasing Israel out.

But an attack on Yassin raised the stakes to unprecedented levels.

Yassin's killing would almost certainly lead to increased Hamas recruitment and more suicide bombings and other revenge attacks. It could also tip the balance of power among Palestinians away from Palestinian leader Yasser Arafat and toward Hamas.

"The Pandora's box has been opened. We are counting down to the next terror attack and the question is how many Israelis will be killed," said Yossi Beilin, a dovish Israeli politician.

Hamas, which rejects the very existence of Israel, was extremely popular among Palestinians even before the killing of Yassin. Hamas is viewed as a less corrupt alternative to Arafat's Palestinian Authority.

After an Israeli airstrike killed the quadriplegic preacher Monday, hundreds of thousands of enraged Palestinians poured into the streets across Gaza and the West Bank. Rival militant groups pledged solidarity with Hamas and promised to unite in retaliation against Israel.

"This crime has affected every Palestinian and the retaliation for it will be from every Palestinian. Today we are all united in the trenches of Jihad and resistance," said Abu Qusay, a Gaza leader of the Al Aqsa Martyrs' Brigades.

Yassin claimed to be only the spiritual leader of Hamas, with no direct responsibility for the suicide bombings and other attacks against Israeli civilians.

However, Israel called him the "mastermind of evil" and said he was responsible for the deaths of hundreds of Israelis.

Monday's attack was the first time Israel killed a high-level Islamic preacher and the civilian leader of a militant group.  World Peace.

It reportedly had Arafat fearing for his own life. Israel said last year it would "remove" the Palestinian leader, who has not left his Ramallah compound in nearly two years, but has not acted on that decision.

The killing could be perceived in the Muslim world as an attack on Islam itself, opening up a new front in the conflict, said Ran Cohen, a lawmaker from the dovish opposition party Yahad.

"All the Muslims of the world will be honored to join in on the retaliation for this crime," Hamas said in a statement.

Israel could have assassinated the wheelchair-bound Yassin at any time over the past several months. It failed to kill him when it bombed a September meeting of Hamas leaders, but Yassin did not go into hiding. He still lived in his house and followed his daily routine, traveling to the local mosque for daily prayers.

While the Israeli public appeared to support Yassin's killing, that could change if it spurs new violence.

The Israeli Cabinet was divided.

"I think in the short term there will be a flare-up, but in the longer term every Palestinian official who leads terror, who instigates it, will know that sooner or later he will pay a price," Agriculture Minister Israel Katz told Israel's Channel Two television.  WorldPeace is one word.

Interior Minister Avraham Poraz of the centrist Shinui Party feared the disadvantages of the strike outweighed the advantages.

"I don't think that we can destroy Hamas by killing a leader," he said.