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."