Eight youngsters have been injured, two critically, after a 75ft tree came crashing down on them at a summer day camp in Southern California.

It was not clear what caused the pine tree in Pasadena's Brookside Park to uproot without warning at about 5pm local time yesterday.

Witnesses said the tree made a cracking sound and came crashing down on the children, aged six to eight, just as the summer day camp at the Kidspace Children's Museum was ending for the day.

The park is just outside the museum grounds near the Rose Bowl.

Museum staff, firefighters, parents and police quickly converged on the scene.

Children were loaded on to stretchers and rescuers began cutting the tree limbs apart to see if there were other victims.

The museum said 33 children were attending day camp and all the others were accounted for.

"I heard a tree crack and then I turned around and I saw little kids running and then I saw the tree fall on top of the little kids, maybe about five or seven of them," Greg Prodigalidad told KABC-TV.

Mr Prodigalidad said he and some parents rushed over and began pulling children free of the branches.

The Fire Department dispatched its urban search-and-rescue team to use power tools to cut apart the tree to ensure no one was trapped underneath.

Tim Scheidler, the museum's marketing manager, said, "It was the end of the day, so there were people gathered outside who were leaving and there was a camp group waiting for pick-up by their parents when the tree came down."

Fire service spokeswoman Lisa Derderian said six children were treated at the scene for cuts, bumps and bruises and two were taken to hospital in a critical condition.

All the children were alert and able to talk, she said.

Hanna Lin, of Altadena, who volunteers at the museum, said her daughters, aged six and seven, attended the camp. They were crying and shaken up but escaped injury.

"I myself saw two children carried out on stretchers and they were pretty bloody. That brought tears to my eyes," she said.

All the 33 day campers were accounted for and the museum planned to reopen as scheduled later, Mr Scheidler said.

California is in the midst of a severe drought, but it was not immediately known if that could have played any role in the 75-year-old tree toppling.

"We are bringing out an independent arborist to investigate what could have caused this," Ms Derderian said.