Two people, including a 19-year-old woman, are in a critical condition and 30 others are injured after a bus crashed into a tree.
The number 59 double-decker bus swerved to avoid another vehicle and hit the tree in Kennington, south London, near the Imperial War Museum shortly before 11am.
The tree was embedded in the front of the bus and emergency crews had to use cutting equipment to gain access to one side of the vehicle to release two people who were trapped.
Two police officers were among those on board when the bus crashed.
Jesse Fox, an actor from north London, was on the top deck of the bus and described the tree crashing through the front windows, pinning two passengers in their seats.
"I was on the top deck and luckily I was three seats from the back," he said. "We were going along at a fairly normal speed and I heard people shout, and I could see out of my right hand window that it was swerving to avoid a car.
"And then I heard people shout a lot louder and looked up and literally saw this tree crash through the front of the bus. Most people on the top deck were fine but for the people at the front, it looked pretty bad.
"There were two people trapped on the top deck when I left, which was about an hour afterwards. The emergency services were trying to get them out.
He explained: "The tree came basically right through the front, into the seats. The whole front of the bus was trashed. I think a lot of the people on the lower deck - injured and not injured - got off very quickly because the doors were opened very quickly.
"The 15 or so of us on the top deck who hadn't been seriously injured were on there for about 10 minutes.
"The emergency services got there really quickly and put up a ladder and we climbed down, apart from the people who were really seriously injured.
"There was one guy who seemed like he was trapped. It looked like his legs and ribs were trapped. He was two seats from the front. The woman who was one seat from the front, all I could see was the back of her head, but she was badly trapped.
"She was caught between the tree and the bus, which was caved in. They were both bleeding."
London Ambulance Service said it had treated 32 people, including seven who were seriously hurt.
London Fire Brigade station manager Clive Robinson said: "Our crews on arrival found a very challenging incident. We had people injured on both floors of the bus.
"We moved one lady from the ground floor who was injured, we moved people from the top floor using ladders and the back door on the bus.
"We've used hydraulic cutting equipment to release two people who were trapped. In addition, the floor of the bus was starting to collapse and we had to shore that up using hydraulic equipment to get the people upstairs out safely."
One witness, a paediatrician who did not want to be named, said she treated people for shock, cuts and bruises and facial injuries.
The front of the bus was crumpled against a tree near the Ship pub in Kennington Road.
Chief Superintendent Glyn Jones said it is not yet clear what caused the crash, but confirmed that no emergency vehicles were involved.
He said: "At this stage we don't know what has caused that collision. That is subject to an ongoing investigation."
The bus "swerved to avoid another vehicle and struck a tree" at around 10.50am, Transport for London's director for buses, Mike Weston, said.
He added: "The incident took place on Kennington Road near the Imperial War Museum, SE1.
"Our first concern is, of course, for the driver, passengers and others involved in this incident. There will be a thorough investigation into this incident as a matter of urgency."
The road is currently closed in both directions with routes 3, 59, 159 and 360 on diversion.
The vehicle involved in the crash is operated by Arriva.