GAZA CITY, Gaza Strip- Hamas founder Sheik Ahmed Yassin was assassinated by an Israeli missile.

Yassin was the most prominent Palestinian targeted by Israel in three and half years of fighting. Hamas has killed hundreds of Israelis in attacks since it was founded in 1987. Hamas' desire is to destroy the Jewish state and set up an Islamic one instead of it.

As ordinary Palestinians got angry and militants pledged retaliation ?including threats against the United States. "We will get revenge for every drop of blood that spilled," said Salman Bdeiri, a Hamas supporter crying near the mosque where Yassin prayed shortly before being killed by an Israeli airstrike.

Israel sealed off the West Bank and Gaza, banning Palestinians from Israel, and placed its security forces on high alert.

Later Monday, Palestinian militants fired several homemade rockets and mortar shells at Israeli targets in and near Gaza. To the north, Hezbollah guerrillas fired an anti-tank missile at Israeli troops along Israel's border with Lebanon.

Israel responded to both attacks. It struck at Hezbollah with planes and artillery fire. No casualties were reported. Israel sent tanks into northern Gaza near the town of Beit Hanoun. Israeli security officials said the purpose of the operation was to prevent further rocket fire.

The Yassin assassination was part of Prime Minister Ariel Sharon's effort to crush Hamas ahead of a possible Israeli withdrawal from Gaza. However, the killing was seen as a major gamble that could galvanize the Palestinians behind Hamas. Rival Palestinian militant groups immediately pledged solidarity with Hamas.

The missile strike also made the relationship between palestine and U.S., and other mdderate Arab states, including Egypt and Jordan, moderate Arab states difficult.

Monday, Hamas threatened the United States. "The Zionists didn't carry out their operation without getting the consent of the terrorist American administration and it (the United States) must take responsibility for this crime," Hamas said in a statement. "All the Muslims of the world will be honored to join in on the retaliation for this crime." It is their first threatening against the United States.

While not condemning the assassination, the State Department said it would increase tensions and make it harder to pursue peace in the Middle East.

But Israel said it will press ahead with more targeted attacks and raids. "The war against terror has not ended and will continue day after day, everywhere," Sharon said. He called Yassin the "mastermind of Palestinian terror" and a "mass murderer who is among Israel's greatest enemies."

More than 200,000 Palestinians, some carrying billowing green Hamas flags, poured into the streets of Gaza City for Yassin's funeral procession. Tens of thousands of furious Palestinians rallied across the West Bank. Mourners in Gaza jostled to touch Yassin's flag-draped coffin, and women ululated and threw flowers and candy. Two Israeli helicopters flew above.

In addition to Yassin, 12 Palestinians, including his bodyguards and relatives, were killed Monday.

The Palestinian Authority declared three days of mourning and closed schools. Flags at Yasser Arafat's headquarters in the West Bank town of Ramallah headquarters were lowered to half-staff.

European and Arab leaders condemned the killing. Egypt canceled a trip by legislators and other dignitaries to Israel to mark the 25th anniversary of the peace treaty between the two countries.