Promising bloody retaliation, thousands of Palestinians demonstrated throughout the West Bank to protest Israel's assassination of Hamas founder Ahmed Yassin (searchin the Gaza Strip early Monday.

The Israeli army imposed a closure on the territory early Monday, fearing Palestinian retaliation. The closure kept Palestinians out of Israel and confined most residents to their communities. Still, sporadic fighting with soldiers was reported in several areas.

Some 15,000 people, including more than 40 armed men, gathered in the center of Nablus, the West Bank's largest city and a stronghold of militants. About 15 armed men, wearing masks and Hamas (search) headbands, fired shots into the air.

"Dozens of people came to us this morning volunteering to be suicide bombers," said one masked militant. "We will send them in the right time."

In Jenin, another militant stronghold, more than 10,000 people demonstrated. Several dozen armed men from the Al Aqsa Martyrs' Brigades (search), a militant group loosely linked with Yasser Arafat's Fatah faction, joined the crowd. The militants held a mock coffin and fired shots into the air.

Calls for revenge emanated from mosque loudspeakers. One Hamas activist said that a new phase in the Israeli-Palestinian fighting had begun.

"What you have gone through now is nothing compared to what you will experience now," he said. More than 450 Israelis have died in homicide bombings over the past three and a half years, many of them carried out by Hamas.

Shopkeepers called a one-day strike throughout the West Bank, closing virtually all stores. Palestinian schools were closed.

Arafat condemned the assassination, saying Israel had "crossed all red lines," and declared a three-day mourning period for Yassin. Flags at his headquarters in Ramallah were lowered to half-staff.

More than 2,000 enraged protesters marched to Arafat's headquarters, chanting calls for revenge. "We sacrifice our souls and our blood for you Sheik Yassin," they screamed.

The crowd knocked on the gate of the headquarters and demanded to speak to the Palestinian leader. Arafat remained holed up inside, apparently fearing that he too might be targeted by Israel.

"He is like a man who was hit on the head because they killed Yassin and now they could kill him," said an aide to Arafat, speaking on condition of anonymity. "He feels his turn is next and he is sad and worried."

Clashes between Palestinian stonethrowers and soldiers were reported in the city of Hebron and town of Bethlehem. Witnesses said the army imposed a curfew and fired tear gas in Hebron, where about 500 Jewish settlers live in a tense enclave surrounded by tens of thousands of Palestinian residents.

In Bethlehem, two people were injured, witnesses said. The army had no information on either incident.

Meanwhile, a Palestinian man got out of a car in the Tel Aviv suburb of Ramat Gan and attacked a group of Israeli pedestrians with a blunt instrument, lightly wounding three people, police said.

The man, a resident of the West Bank village of Bidya, was subdued and arrested. Police said the motive was revenge for the Yassin killing.

Yassin was killed with seven other people early Monday as he left a mosque in the Gaza Strip. The killing also sparked larger, more violent demonstrations throughout the Hamas stronghold of Gaza.