By Jo Floto
BBC News, Jerusalem
The driver of the bulldozer repeatedly rammed the bus full of people
It is just before midday. Jerusalem's main East-West thoroughfare, Jaffa Road, is extra busy.
One lane has been closed for weeks while the city lays track for a light railway. On the building site a man is about to drive a huge bulldozer into cars and into people.
The BBC's offices on Jaffa Road are three floors above street level. They're pretty well sound-proofed but correspondent Tim Franks is alerted to events below by screams coming from below his window.
Looking out he sees the bulldozer repeatedly ramming a bus full of people until the bus turns on its side.
Shouts and screams
We rush down the stairs and onto the street following the bulldozer. In its wake it has left crushed vehicles, dead and wounded people, and hundreds of shocked bystanders.
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Some of the wounded are being helped from the bus, others cry out for help as fuel spills from the engine.
To our right is a woman in a white car. The front of the vehicle has been flattened. She is dead, although we hear later that her child has been saved.
By the time we get to the bulldozer it has stopped, a man comes up to us with his pistol drawn.
He is Shmuel Elkayam, a security guard from a nearby bank. He says he has just killed the driver. He hasn't.
As we speak the engine of the bulldozer roars as it sets off again, heading towards the city's busiest market packed with thousands of shoppers. Shouts and screams ring out.
Three men have jumped onto the cab - two policemen and a civilian - trying to prise open the door and lift the man's leg from the accelerator.
There was no immediate claim of responsibility for the attack
After a few seconds the door is opened enough to get an arm, and a pistol, into the cab. The driver is then shot in the head.
Later the police identify him as a 30-year-old Palestinian father of two, a resident of east Jerusalem. His motivation is unclear - whether personal or political.
Three Palestinian militant groups have claimed responsibility but at the moment it appears that the man was acting alone. What is certain is that he was trying to kill as many people as possible.
Apart from a gun attack in March, Jerusalem has seen relatively little violence in the last three years.
Since the start of 2008, across Israel and the occupied territories, 29 Israelis have been killed in violence linked to the conflict as have more than 400 Palestinians.