The sounds of bulldozers echoing beneath him, veteran tree sitter John Quigley perched in a century-old oak on Wednesday, saying he won't come down until public works officials stop felling scores of trees as part of a dam improvement project.

Protester Camron Stone speaks during a candlelight vigil in support of tree sitters who are attempting to keep oak trees from being felled in Arcadia, California. [AP]

Quigley, who helped save a beloved oak with a similar sit-in eight years ago, was joined by a handful of other sitters who took up positions in branches overlooking 4.45 hectares of picturesque foothills.

"They're destroying trees all around us," Quigley said by cell phone as the sound of bulldozers below him could be heard. "It's a sad scene and definitely something that didn't need to happen."

Public works officials say the grove of trees, some of them more than 100 years old, must go to ensure the integrity of a nearby dam that provides most of the drinking water to the Los Angeles suburbs of Arcadia and Sierra Madre.

As darkness fell about 2,000 protesters and onlookers including actress Darryl Hannah gathered at a gate leading to a stand of trees being felled.

"I came out just to support the community that is trying to put out some common sense and not cut down a paradise for a rubble pit," Hannah said.

The actress said she learned of the protest from Quigley, who she has known since she took part in a tree-sitting protest to try to save an urban garden in a warehouse district near downtown Los Angeles that was plowed under in 2006.

Hannah, like other environmental activists, said the sediment from the Dam could be placed elsewhere.

Later on Wednesday about three dozen people held a candlelight vigil with a moment of silence to express their dismay over the removal of the trees.

Los Angeles County Public Works spokesman Bob Spencer said four people were believed to be hiding in the trees and public workers were checking the area tree by tree to make sure none was taken down with a person in it.

David Czamanske, vice-chair of the Sierra Club's Pasadena group, said deputies had not asked the demonstrators at the gate to disperse. The tree-sitters were not affiliated with his group, he said.

Spencer said the tree removal project has been in the works for three years and the county has approval from federal and state agencies. He said it must be done for the Santa Anita Dam, which was built in 1927, to meet seismic safety standards.

Over the years, Spencer said, sediment has built up behind the dam, limiting its water capacity and compromising its safety in the event of an earthquake or other catastrophe.

Clearing the 4.45 hectares of oaks and sycamores will create a placement area the sediment can be channeled to.

Spencer said the dam provides 75 percent of the drinking water used in Arcadia, a city of about 56,000 people, and all of the drinking water for Sierra Madre, where about 10,000 people live.

The grove occupies a flatland below the steep slopes of the San Gabriel Mountains, about 32 km northeast of downtown Los Angeles.

Czamanske and Quigley agree the sediment removal project must go forward, but they say the county should have picked a better place.

"It really is a tragedy that they had to go to this beautiful habitat to dump a pile of mud," Quigley said. "There were plenty of good alternatives."