JERUSALEM — A Palestinian Arab man plowed an enormous construction vehicle into cars, buses, and pedestrians on a busy street today, killing at least three people and wounding at least 45 before he was shot dead by security officers.

Israeli police referred to the man as a “terrorist” acting on his own. He repeatedly smashed vehicle after vehicle with the huge shovel on his machine, throwing cars into the air and overturning a bus.

The first major attack in Jerusalem since March wreaked havoc in the heart of downtown. Hundreds of people fled in panic through the streets as medics treated the wounded.

Three Palestinian Arab militant groups took responsibility for the attack, but the claims could not be independently verified.

The attack took place in front of a building housing the offices of The Associated Press and other media outlets. British Broadcasting Corp. footage captured the huge front loader crushing a vehicle and an off-duty soldier shooting the perpetrator in the head several times at point-blank range as onlookers screamed.

Israel’s national rescue service confirmed three deaths, and the bodies lay motionless on the ground covered in plastic.

A 19-year-old soldier, Hen Shimon, said the whole scene was a “nightmare.”

“I just got off the bus and I saw the tractor driving and knocking everything down in his path,” she said. “Everything he saw he rammed.”

Yosef Spielman said the construction vehicle picked up a car “like a toy.”

“I was shocked. I saw a guy going crazy,” he said. “All the people were running. They had no chance.”

Eyal Lang Ben-Hur, 16, was in a bus when the driver yelled “Get out of the vehicle! Everyone out!” People fled in a panic, he said, and the bus was hit an instant later.

An officer in a special anti-terror unit, Eli Mizrahi, said he and his partner sped to the scene on a motorcycle from the nearby Mahane Yehuda market at downtown Jerusalem. An off-duty soldier had just shot the attacker, but not killed him.

“I ran up the stairs (of the vehicle) and, when he was still driving like crazy and trying to harm civilians, I fired at him twice more and, that’s it, he was liquidated,” Mr. Mizrahi told reporters.

The attack occurred in an area where Jerusalem is building a new train system. The project has turned many parts of the city into a big construction zone. Israeli police said the attacker was a bulldozer operator who worked in the area for a construction firm.

During the second Palestinian Arab uprising, which erupted in late 2000, Jerusalem experienced dozens of suicide bombings and other attacks. The city has been largely quiet in the past three years, though sporadic attacks have persisted. In March, a Palestinian Arab gunman entered a Jerusalem seminary and killed eight young students.

An Israeli government spokesman, Mark Regev, called today’s attack a “senseless act of murderous violence.”

President Bush called Prime Minister Olmert to express condolences about the attack and planned to call the president of the Palestinian Authority, Mahmoud Abbas, as well, the White House press secretary, Dana Perino, said.

The three organizations that took responsibility for the attack included the Al Aqsa Martyrs’ Brigade, which is affiliated with Mr. Abbas. An Abbas aide, Saeb Erekat, condemned the violence.

The other two are the Galilee Freedom Battalion, which is suspected of being affiliated with Lebanon’s Hezbollah guerrillas, and the Popular Front for the Liberation of Palestine, a fringe left-wing militant group.

The Hamas militant group, which runs the Gaza Strip and is currently maintaining a fragile cease-fire with Israel, said it did not carry out the attack but nevertheless praised it.

“We consider it as a natural reaction to the daily aggression and crimes committed against our people in the West Bank and all over the occupied lands,” a Hamas spokesman, Sami Abu Zuhri, said.

Despite the Palestinian Arab claims of responsibility, the Israeli police chief, Dudi Cohen, said the attacker appeared to be acting alone. “It looks as if it was a spontaneous act,” he said.

Major Israeli retaliation appeared unlikely given the police chief’s claim that the attacker acted alone and the Jewish state’s desire to maintain the Gaza cease-fire and to support Mr. Abbas’s security forces in the West Bank.

Israeli police said the man, a father of two children, was an Arab in his 30s from east Jerusalem and had a criminal background. A police spokesman, Micky Rosenfeld, said he had worked as a construction worker on the railway project.

Later today, five military vehicles gathered outside the man’s two-story home at east Jerusalem. Police entered the house, took pictures and questioned relatives for an hour before leaving without making arrests.

Friends of the family identified the attacker as Hussam Dwikat. They said the 29-year-old was a devout Muslim, but had no known ties to any militant groups.

“Everybody is in shock. When I was told what happened I started to curse Hussam because this is the first time he has done something like this,” a friend of the attacker’s wife, Salayan Weyed, said.

In contrast to West Bank Palestinian Arabs, Arab residents of Jerusalem have full freedom to work and travel throughout Israel. About two-thirds of Jerusalem’s 700,000 residents are Jews, and the rest are Palestinian Arabs who came under Israeli control when Israel captured their part of the city in 1967.

Israel’s national rescue service said at least 45 people were wounded in today’s attack. Wounded people sat dazed on the ground amid piles of broken glass and blood stains on the street. A half-dozen cars were flattened and others were overturned by the Caterpillar vehicle. A bus was overturned and another bus was heavily damaged.

A woman sprinkled water over a baby’s bloodied face, a rescue worker stroked the hair of a dazed elderly pedestrian and a loved one raised the bleeding leg of a woman sitting outside the overturned bus.

A 52-year-old pedestrian, Esther Valencia, said she barely escaped the carnage.

“He almost hit me. Someone pushed me out of the way at the last moment. It was a miracle that I got out of there.”

The office manager for AP’s Jerusalem bureau, Cassia Pereira, watched the attack unfold outside her window.

“I saw him but it was too late and there was nothing to do,” she said, with tears in her eyes. “I was in panic I couldn’t say a word … I realized something was not normal, something was wrong.”

The mayor of Jerusalem, Uri Lupolianski, said his daughter was on one of the buses rammed by the attacker, but she was not injured.

“To our regret the attackers do not cease coming up with new ways to strike at the heart of the Jewish people here in Jerusalem,” Mr. Lupolianski said.