Hours after the assassination, Hamas made an unprecedented appeal for help from "all the Muslims of the world" in carrying out reprisals. The group also sent a threat to the United States, saying it held Washington responsible for Yassin's death because of its support for Israel.

In the past, Hamas leaders — Yassin foremost among them — insisted their struggle is only against Israel and that they would not get involved in causes by militant Muslims in other parts of the world or attack U.S. targets.

In recent months, Hamas has cooperated more closely with the Hezbollah guerrillas, and the Lebanese group was seen as a possible arm for revenge, in case Hamas has difficulty staging an attack because of Israel's clampdown.

It would also be easier for Hezbollah or other Muslim militant groups to strike U.S. or Jewish targets worldwide.

On Monday evening, Hezbollah guerrillas fired an anti-tank missile at Israeli troops along Israel's border with Lebanon, and Israel responded with an airstrike and artillery fire, the army said.

The killing of Yassin dealt what could be the final blow to the moribund U.S.-backed "road map" peace plan. It appears unlikely Palestinian Prime Minister Ahmed Qureia would now agree to meet with Sharon; such a summit is a prerequisite for moving ahead with the plan.

Hamas' show of strength Monday — hundreds of thousands participated in Yassin's funeral — shook the Palestinian Authority, whose popularity has plummeted during the current fighting.

"In every city, in every village, people were chanting Hamas slogans," said Hani al-Masri, a Palestinian journalist.

Palestinian leader Yasser Arafat quickly tried to co-opt his longtime political rival in death, praising Yassin as a "martyr" and opening a house of mourning at his West Bank headquarters in Ramallah.

Yassin's death was not expected to disrupt Hamas' ability to carry out terror attacks. The group is tightly organized and takes its cues from leaders based in Syria and Beirut. Yassin was a chief ideologue and, according to Israel, had a say in most attacks, but apparently was not closely involved in planning.

"Hamas suffered a severe blow, but more to its morale than its operational side," Reuven Paz, an Israeli counterterrorism expert, told Israel Radio. "I think that its ability to recover, with the help of the massive support it will now receive from the Palestinian street, will be very quick."

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Karin Laub has been news editor in the AP Jerusalem bureau since 1995.