OAKLAND — Thousands of environmental and labor activists took to the streets of Jerry Brown’s hometown Saturday carrying signs and chanting slogans with a simple message for the governor: end fracking.

The marchers called on Brown to “stop fracking” with people’s health and “follow the science,” saying the governor’s legacy as an environmental leader was at stake if he didn’t ban the practice.

“The governor has done a lot of really good things for the environment,” said Richard Gray, 71, a retired landscape designer from Marin County. “But fracking seems to be a blind spot for him.”

Gray held a sign with an image of the governor pointing to his head and the words, “Climate Leaders Use Their Brains.”

California is the No. 3 oil producer in the nation and has added an average of 300 wells each month for the past decade. About half of them are using hydraulic fracturing or fracking, a technique that involves injecting water, sand and chemicals to break apart underground rocks so oil and gas can escape.

Two large buses took protesters from Sonoma County to Oakland for the demonstration.

Environmentalists say numerous studies show fracking is worsening climate change, exacerbating California’s historic drought and jeopardizing the health of residents. They have clashed with Brown, who was a darling of the environmental movement in the 1970s when he ended tax breaks for oil companies and promoted solar energy, but had his name interchanged with Chevron and big oil during a chant at Saturday’s protest.

Brown’s administration says that so far there is no direct evidence of harm from fracking in California. The state has “the most imaginative and aggressive and integrated strategy to deal with climate change of any political jurisdiction in the Western hemisphere,” the governor said at a news conference on Friday.

But Brown said the state’s residents continue to consume oil, so either California has to produce it itself or get it elsewhere.

Sabrina Lockhart, a spokeswoman for Californians For Energy Independence, a coalition that includes energy companies, echoed Brown’s comments, saying any reduction in oil production in California would increase the state’s dependence on imported oil, which creates its own risks and effects when it’s transported.

“These activists are using fracking as a scare tactic when the end goal is to end all oil production in California,” she said.

Buses brought protesters from across the state to a plaza in front of City Hall in downtown Oakland for Saturday’s rally and march. Organizers of what was dubbed the “March for Real Climate Leadership” had expected 10,000 protesters, and estimated the actual crowd at around 8,000 based on aerial surveys along the march route and an analysis of time-lapsed video of the march.

Oakland police gave a much lower estimate of as many as 2,000 protesters. They did not report any arrests or citations.