The 17-year-old warned that, despite the “beautiful words and promises“ from elected officials, not enough was being avert a climate catastrophe.
“I will not be silenced when the world is on fire,” she told the crowd at the Bristol Youth Strike 4 Climate Change.
The teenager was greeted to chants of her name from the crowd, many of them school pupils who have been taking part in regular walkouts to protest government inaction on global warming.
Greta, who arrived at Bristol Temple Meads train station on Friday morning, went on to join protestors in a march through the city centre.
Police had initially warned the event could be overrun by supporters, leading to safety issues. In a response, the youth strike’s organisers said they had “no time to be patronised”.
Taking aim at politicians, Greta added: “Once again, they sweep their mess under the rug for us – young people, their children – to clean up for them.
“But we must continue and we have to be patient. Remember that the changes required will not happen overnight since the politics and solutions needs are far from sight.”
The teenager said she had chosen to appear in the southwest city due to the environmental movement being “particularly strong here”.
Her speech marked the high point of the 10th youth strike to be held in the city, a format emulated across the globe and inspired by Greta’s initial protests in which she sat in solitude outside the Swedish parliament in 2018 to pressure the government into action.
“We will not be silenced because we are the change, and change is coming whether you like it or not,” she told the crowd.
“This emergency is being completely ignored by the politicians, the media and those in power.
“Basically, nothing is being done to halt this crisis despite all the beautiful words and promises from our elected officials.
“So what did you do during this crucial time? I will not be silenced when the world is on fire.”
She was preceded on stage by Mya-Rose Craig, the 17-year-old birdwatcher and environmentalist who earlier this month became the youngest person to receive an honourary doctorate from the University of Bristol.
“We have to engage with all of our communities in order to properly fight climate change.” she said.
“An unequal world can never be a sustainable one.”
Additional reporting by PA