London (dpa) - The global establishment is offering only "beautiful words" to mitigate the climate crisis, Swedish teenager Greta Thunberg told British supporters.

"How do we want to be remembered?" Thunberg, 17, asked thousands of activists at a Fridays for Future climate change protest in the south-western English city of Bristol.

"This is an emergency ... and it will get worse," she told the crowd, which police estimated at more than 20,000.

"This emergency is being completely ignored by the politicians, the media and those in power," Thunberg said in a brief speech before the protesters marched through the city.

"Basically, nothing is being done to halt this crisis despite all the beautiful words and promises from our elected officials," she said.

"So what did you do during this crucial time?" Thunberg added. "I will not be silenced when the world is on fire."

Arriving at Bristol's main railway station earlier, Thunberg told the local Bristol Post that she had chosen to speak in the city for "many different reasons."

"The movement is very strong here and I had contact with people who were here," she said.

City mayor Marvin Rees tweeted his welcome to Thunberg and "all the young people who've come to Bristol today," saying his maritime city is at "the forefront of action on climate and social justice."

"It's incredible for Bristol to welcome Greta to our city because she has inspired millions of young people around the world to strike for a better future and for governments and nations to take climate action in the face of dangerous climate change," Eunice Lo, a climate scientist at Bristol University, said in a video before the protest.

Bristol and the university were the first in the Britain to declare a climate emergency, Lo said.