Greta Thunberg criticised officials and governments as she told thousands of protesters in Bristol that "they are behaving like children, so it falls on us to be the adults in the room".
The 17-year-old activist was the headline speaker at The Bristol Youth Strike 4 Climate on College Green today (February 28).
Police said more than 20,000 people attended the event, although organisers said they estimated the figure to be closer to 30,000.
Greta took aim at officials as she delivered her speech, saying: "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 took to the stage to chants of "Greta, Greta" from the schoolchildren who had gathered in the rain.
She told them: "We will not be silenced because we are the change, and change is coming whether you like it or not.
"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."
The Swedish teenager went on to join protesters on a march through Bristol city centre accompanied by a samba band.
Avon and Somerset Police have warned of the potential for protesters to be hurt from "tripping, falling or being crushed by the crowds" given the anticipated popularity of the event.
In a joint statement, Mike Jackson, executive director of Bristol City Council, and Superintendent Andy Bennett, Bristol police commander, warned of "major disruption" from the event.
"Bristol City Council and the Avon and Somerset Police are unable to accurately predict how large this event will be, however we have confirmation of people travelling from across the UK to attend," they said.
They urged people to allow extra time for travel and consider whether their journey was necessary.
In a response posted on Twitter, Bristol Youth Strike 4 Climate said they had "no time for being patronised".
Organisers said they were "honoured" to be welcoming Greta to Bristol for their 10th strike.
