Thousands of Bay Area students walked out of schools Friday to protest climate change and many of them marched down Market Street in a noisy, energetic show of youth frustration and determination.

“Climate change is not a lie! We won’t let our planet die!” the young people shouted, amid a sea of signs, banners and a giant beach ball painted to resemble the planet that many protesters said was on life support.

It was all part of an international day of youth protest over the state of the Earth. Similar protests took place across the U.S. and in more than 100 countries.

In San Francisco, about 2,000 students — many with their teachers’ blessing — and their parents gathered first in front of Rep. Nancy Pelosi’s office at 7th and Mission streets, then marched to Sen. Dianne Feinstein’s office at Market and Post streets before heading for a rally at Union Square. Traffic came to a halt. Some motorists honked, not all in support.

“There is no planet B, C, D,” Isha Clarke, a student leader of Youth vs. Apocalypse told the crowd. “The cycle of destruction ends now.”

Handmade signs held aloft by the young marchers stressed the dire consequences of climate change and criticized inaction by adults.

“It’s not nice to frack Mother Nature,” said one sign.

“The seas are rising and so are we,” said another.

Claudia Ryckebusch, an eighth grader at Denman Middle School in San Francisco, missed her geology class to join the march. She was holding a sign that said, “Climate change is real,” next to a picture of wilted flowers.

“The planet still has to be here after adults aren’t,” Ryckebusch said. “We’ve got to take care of the planet before it’s gone forever. I don’t want my great-great-grandchildren to live in a sea of trash.”

Morgan Swanson, a 15-year-old ninth-grader at Bright Works School in San Francisco, said he was marching “for my future.”

“If things don’t change, I’m going to have to put on a protective suit just to walk outside,” he said. “I don’t want to have to do that.”

Jesse Simons marched with his daughter, 8-year-old, Eloise.

“Her future is on the line,” he said.

Eloise, a second-grader from Park Day School in Oakland, held a sign that said, “Everything needs to change and needs to start today.”

“Things are bad and it affects kids more than adults,” Eloise said.

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Some marchers’ signs also called on Feinstein to vote “yes” to the Green New Deal, a proposal by Rep. Alexandria Ocasio-Cortez, D-N.Y., and Sen. Ed Markey, D-Mass., calling on the U.S. to run on 100 percent renewable energy within the next decade.

Feinstein’s terse conversation about the Green New Deal with young environmental activists at her office in February went viral as they debated how she, a Democrat, should vote on the proposal. On Friday, activists dropped posters off in front of her office. Some of the young people who faced off with Feinstein in the video were among Friday’s marchers.

Clarke was one of them. As the crowd assembled around the stage at the north side of Union Square, she bemoaned adults who tell children to remember their place.

“A child’s place is at the table where their futures are being discussed,” she said, as the crowd roared its approval.

“Despite being young, we are literate and capable,” said 12-year-old Samantha May Robles of Oakland. “If you think you can leave us a messed up climate, you’re a fool and you’re a murderer.”

The marchers in San Francisco and around the world were inspired by Greta Thunberg, a 16-year-old Swedish student activist, who started protesting on her own in September 2018 against what she said were adults’ inaction on climate change. Students in Paris, New Delhi and New York City left their classrooms Friday to do the same. In some ways, the Friday protest seemed a ramped-up version of Earth Day, which has been taking place for half a century and is set for April 22.

In San Francisco, police officers on bikes, motorcycles and on foot kept watch over the march as it proceeded down Market Street and through the Financial District to Union Square. Streetcars, trollies, buses and cars backed up. Some Muni lines were rerouted, according to the San Francisco Municipal Transportation Agency.

A spokeswoman for San Francisco Unified School District said that, as of noon, about 240 students had left class to join the strike, which ended early in the afternoon. Many were from the Ruth Asawa School of the Arts and The Academy high schools.

Steve Rubenstein, Ashley McBride and Gwendolyn Wu are San Francisco Chronicle staff writers. Email: srubenstein@sfchronicle.com, ashley.mcbride@sfchronicle.com, gwendolyn.wu@sfchronicle.com Twitter: @SteveRubeSF @ashleynmcb @gwendolynawu