Greta Thunberg has warned world leaders she will “not be silenced when the world is on fire”.
The teenager was welcomed by chants of “Greta, Greta” as she addressed some 20,000 people at the Bristol Youth Strike 4 Climate (BY24C) event.
She accused politicians of ignoring the climate emergency and “sweeping their mess under the rug”.
“We are the change, and change is coming whether you like it or not,” the Swedish environmentalist said.
Greta was speaking at the climate strike event on College Green, before leading the march through the city.
“Activism works so I’m telling you to act,” she said. “We are being betrayed by those in power.”
Wet weather failed to deter thousands of people turning out for the 17-year-old, who appeared on stage at about 11:45 GMT.
“Our leaders behave like children so it falls to us to be the adults in the room. They are failing us but we will not back down,” Greta told the crowds.
“It should not be this way but we have to tell the uncomfortable truth. They sweep their mess under the rug and ask children to clean up for them,” she said before setting off on the march.
Police said 20,000 are in attendance, although organisers believe the figure is closer to 30,000.
Annie, a 21-year-old Bristol University student, was of the many people who turned out to hear the headline speaker and has taken a day off from her studies to participate.
“This is probably one of the most important things that we should be focussing on right now. Showing support and that you want change will make people higher up realise that as well.
“One day off school is a good enough sacrifice for what this is standing for. It’s just one day, it’s not going to do any harm.”
Mya-Rose Craig, the youngest person to be awarded an honorary doctorate from the University of Bristol, spoke to the crowd before Greta’s arrival on stage.
She said: “Greta, welcome to our amazing city and thank you for being with us today. We have to engage with all of our communities in order to properly fight climate change.
“An unequal world can never be a sustainable one.”
BYS4C said it had drafted in more than 80 stewards and has a sectioned-off a “safe zone” for young children as well as an accessibility area.
Avon and Somerset Police and Bristol City Council had warned of “major disruption” during the event, but stressed they would be “working to ensure Bristol is open as usual”.
More than 20 road closures are in operation from Broadmead and the Bearpit across to Hotwells, Brandon Hill and Park Street.
Ms Thunberg had originally intended to visit London, but the area planned for the protest in the capital was considered too small so organisers recommended Bristol instead.
Two years ago, Ms Thunberg started missing lessons on Fridays to protest outside the Swedish parliament building, in what turned out to be the beginning of a huge environmental movement.
She has since become a leading voice for action on climate change, inspiring millions of students to join protests around the world.