The protesters are calling for urgent action on climate change, including putting pressure on local elected leaders to support more drastic steps.
In San Francisco, they rallied near House Speaker Nancy Pelosi's office at the Federal Building at Seventh and Market streets and took off marching down Market Street at about 10:30 a.m. chanting, "Climate change has got to go" and "Climate justice now."
RELATED: San Mateo students walk out for global climate strike
They marched to U.S. Sen. Dianne Feinstein's office at 1 Post Street and planned to return to the Federal Building for a rally.
Organizers for the San Francisco action said that they are calling on Pelosi and Feinstein to back the Green New Deal, an ambitious climate action plan introduced by U.S. Rep. Alexandria Ocasio-Cortez, (D-New York.)
They are also calling on corporations like Bank of America, Amazon and PG&E to divest from the fossil fuel industry and improve their own reliance on renewable energy sources.
The San Francisco march is being billed as the largest youth demonstration against climate change ever, according to the organization Youth Vs Apocalypse.
Students participating said that dire forecasts of rising global temperatures made them fearful of the consequences if there is not immediate action.
"I'm scared for our future, if we even have one," said Otto, a 13-year-old student from San Francisco.
Caroline Hench, a high school student from Oakland, said, "I think it's more important to protest against out climate than go to school because we won't even be able to do that soon."
Concerned students waiting to speak at a climate strike at San Mateo High School. pic.twitter.com/3cGB367XDk— Amy Hollyfield (@amyhollyfield) September 20, 2019
Tracey Cook, also from Oakland said, "I think it's really important the teens' voices gets heard because a lot of times it gets put to the side. So it's important we're here to show we have a voice and be heard."
A spokesperson for Berkeley Unified School District said that 400 students walked out of Berkeley High School and took BART to the San Francisco action.
Many students from Oakland schools were attending the rally on field trips, district spokesperson John Sasaki said.
Students are also expected to walk out of the University of California at Berkeley. As of about 11 a.m., they were gathering at Sproul Plaza.
Walkouts and rallies are also planned in Cupertino, Los Altos, Oakland, Palo Alto, Richmond and in more cities across the region. Students at American High School in Fremont planned to have a lunchtime rally.
They’re walking out of class right now at San Mateo High School for a climate protest. Students say they are demanding that the government take action against climate change. pic.twitter.com/Vtidmn5qbt— Amy Hollyfield (@amyhollyfield) September 20, 2019
Striking students in Marin County gathered at 9 a.m. at the San Rafael City Center at 1002 4th St. In Mill Valley, students gathered downtown at 11 a.m. at Depot Plaza at 87 Throckmorton Ave.
In Sonoma County, students gathered at the Central Quad Santa Rosa Junior College at 9:30 a.m. and plan to march to a noon rally at Courthouse Square in downtown Santa Rosa.
Other rallies were held across the nation and world, with large rallies in Africa, Asia, Australia, Europe and South America.
Actions were planned in 150 countries, including 800 in the U.S. alone. Tens of thousands of people participated in rallies in Berlin, London and New York.
Actions are scheduled to continue through next week.