TEL AVIV -- A bomb shattered a bus in the midst of Tel Aviv's shopping area Wednesday in an apparent suicide attack by Islamic militants. Police said 22 people were killed and 48 wounded.


Israeli leaders warned of retaliation following the blast, one of the bloodiest in Israel's history and the third major attack in 10 days claimed by Islamic militants of the Hamas group.


"We will be very tough on the Hamas," Uri Dromi, spokesman for the government, told reporters. "We must both fight the Hamas and other Islamic fanatics and at the same time pursue the peace process. The only alternative is bloodshed."


Police spokesman Gadi Doron said the bomb was made up of between 10 and 20 kilograms of explosives. Authorities believe the bomb was carried aboard the red-and-white Dan bus No. 5 by a suicide guerrilla.


Police did not rule out the possibility that the bomb was planted on the bus, however.


The explosion ripped apart the northbound commuter bus at about 9 A.M. as it traveled up tree-lined Dizengoff Street in the heart of Israel's largest metropolitan area.


"People went flying into the air, arms and legs, it was a very terrible thing," said Moshe Reiner, who was walking to work when the blast occurred.


The force of the explosion gutted the vehicle, hurling a large twisted piece of metal from the bus onto a power line and other parts onto rooftops.


President Ezer Weizman, who refused to accept a sympathy call from PLO leader Yasser Arafat, lashed out angrily and vowed retaliation.


"It will end. It has to end. We will tear them to bits," he told reporters, adding that Israel would have to take "extraordinary action" to find the roots of the Islamic extremist groups. Wednesday's attack exploded the euphoria surrounding Israel's peacemaking with Jordan and the planned trip of U.S. President Bill Clinton for the signing of a peace treaty next week.


In a statement read out in Gaza mosques, the Izzedine al-Qassam Brigades, the military wing of Hamas, claimed responsibility and vowed to launch more attacks.


"God is great. God will torment them with your hands and the hands of the faithful," the statement said, adding that the attack was in revenge for the killing of three militants who kidnapped and killed an Israeli soldier last week.


The attack came as Israeli and PLO officials sat down in Cairo to discuss expanding autonomy in the West Bank. The talks broke off early because of the blast, and there was no date for resumption.


Israel maintains that the PLO has done too little to rein in Islamic groups, and Israeli Foreign Minister Shimon Peres said it was "too early" to say if Israel would press ahead with talks.


Arafat issued a statement "in the name of Palestinians and in my name" condemning the bombing and extending his condolences to the innocent victims.


The army, meanwhile, imposed a closure on the West Bank and Gaza Strip, barring Palestinians from entering Israel.


Prime Minister Yitzhak Rabin returned to Israel, cutting short a trip to London. He blamed Islamic radicals for trying to destroy the peace process.


It was the bloodiest attack since the 1978 coastal road attack in which 35 Israelis were killed by Palestinian rejectionists who hijacked a bus.