An Israeli bus driver and a paramedic pinned a Palestinian homicide bomber to the ground Thursday after spotting his bomb belt, then fled with other bystanders before the man detonated the explosives. The assailant and an elderly woman were killed, and four people were injured.

There was no immediate claim of responsibility. The Islamic militant group Hamas has said it would avenge an Israeli raid of the Gaza town of Khan Younis this week in which 16 Palestinians were killed.

Also Thursday, two Palestinian boys, ages 12 and 17, were killed in an exchange of fire between Israeli soldiers and local gunmen in the Gaza Strip. The deaths brought to 20 the number of Palestinians killed in Gaza this week.

In the homicide attack, the bomber first tried to board the bus at a crowded bus stop near the Tel Aviv suburb of Bnei Brak, but slipped and fell onto the sidewalk, apparently pulled back by the weight of the explosives. The driver said he got off the bus to help the man and spotted the bomb when he opened the man's shirt to check for injuries.

A Hamas spokesman, Ismail Abu Shanab, said he did not know who was behind the attack, but that "resistance will continue by all means until we get our rights."

Yasser Arafat's Palestinian Authority has condemned attacks on civilians in Israel. However, Arafat adviser Nabil Abu Rdeneh, said Thursday that "the Israelis are responsible for every single act of violence in the region."

David Baker, an official in Prime Minister Ariel Sharon's office, said that "Palestinian terrorists have declared open season on Israelis," and held the Palestinian Authority responsible, saying it did nothing to prevent attacks.

The explosion went off just before 8 a.m. at a bus stop on a major highway east of Tel Aviv. Just before the explosion, the bomber tried to board a bus which was crowded with soldiers.

The Tel Aviv police chief, Yossi Sedbon, said the bomber slipped and fell off the steps, onto the sidewalk.

The driver, Baruch Neuman, got off the bus to help the man, still unaware he was carrying explosives. A paramedic also approached the man lying on the ground.

The driver said he opened the bomber's shirt to check for injuries and spotted the bomb belt. "We pinned down his arms," Neuman said. "There were lots of people at the bus stop. After everyone fled, we released him and he blew up."

Neuman, who was unarmed, said he let go of the bomber because he began to fear for his life. He said the assailant, who had been passive, was beginning to struggle. "We (the driver and the paramedic) made the decision together to let go of the arms, and to flee together," Neuman said.

The bomber then got up and ran for about 90 feet, toward a group of people at the bus stop.

"I saw the terrorist run after us and then I heard the explosion," said a bystander, 45-year-old supermarket cashier Niri Salam.

An elderly Israeli woman was killed and four other bystanders were wounded, police said. About a dozen others were treated for shock.

Many of the passengers were soldiers, and it was not immediately clear why they did not kill the assailant before he blew himself up.

It marked the second time in three weeks that the Tel Aviv area was targeted. On Sept. 19, a Hamas activist blew himself up on a Tel Aviv bus, killing five Israelis.

The pace of homicide attacks has slowed since June when Israeli troops reoccupied most West Bank towns, in response to two deadly bombings in Jerusalem. However, even with the heavy military presence, including curfews that confine hundreds of thousands of Palestinians to their homes, Israel has been unable to seal its frontier with the West Bank completely.

Overnight, Israeli troops entered several West Bank towns and villages and arrested 70 Palestinians. The military said the raid targeted Hamas. In one village, soldiers found and detonated a 33-pound bomb.

In Gaza, Israeli tanks and troops exchanged fire throughout the night and into the morning with Palestinian gunmen in the Rafah refugee camp near the Egyptian border, witnesses said.

Palestinians said more than 20 tanks and several jeeps, backed by attack helicopters, moved on the refugee camp from two directions, firing machine guns. After daybreak, children and teen-agers came out of their house to watch the fighting, residents said, and two boys, ages 12 and 17, were killed.

The Israeli military said that the fighting broke out during an army operation to seek and destroy tunnels used to smuggle arms into Gaza from Egypt. It said two such tunnels were sealed.

A few hours earlier, two Palestinians were killed and 17 wounded when soldiers in tanks opened fire with machine guns at Palestinians throwing rocks at them, witnesses said. The Israeli military said soldiers returned fire after gunmen shot at them.

There are daily clashes along the frontier, which is under Israeli control according to interim peace accords.

Meanwhile, three Israeli border policemen are being investigated for allegedly stealing computer equipment from Arafat's headquarters during a military operation, officials said Thursday.

During a search of the policemen's homes, investigators found a VCR and computer equipment, the Justice Ministry said. The policemen admitted the items were taken from Arafat's compound in the West Bank town of Ramallah, the statement said.

Israel besieged Arafat's headquarters from late March to early May as part of a broad West Bank military offensive against Palestinian militants. Last month, Israeli soldiers kept Arafat under siege for another 10 days following back-to-back suicide bombings that killed six people.

The Israeli newspaper Yediot Ahronot reported that the alleged looting took place during the spring blockade.