Greta Thunberg received another rock star welcome as she joined a school strike in the UK. 

The 17-year-old Swedish activist joined protesters on Friday in Bristol, in the North West of England, after accepting an invitation from the city's Youth Strike 4 Climate group. 

Despite heavy rain, the crowd cheered and waved placards as she called on them to continue to make their voices heard.

"World leaders are behaving like children, so it falls on us to be the adults in the room," she said.

"It should not be this way. We should not be the ones who will have to lead on this and tell the uncomfortable truth. Once again, they sweep their mess under the rug for us young people, for their children, to clean up for them," she added.

The 15,000-strong crowd roared with approval. Some of the young campaigners had traveled miles to see her and she did not disappoint them. She was on home turf.

Bristol was the first British city to declare a climate emergency in 2018 and four weeks ago was the first to declare an ecological emergency as well.

"I thought it was amazing, that's why I came to this – because it's the sort of thing you'll remember," said 15-year-old Sam Robertson. "I feel it's more important watching something like that than being at school."

Bristol aims to be carbon neutral and climate resilient by 2030 – ambitious goals that many fear can only happen with strong national and international leadership.

Before joining a march with demonstrators, Thunberg told the crowd that change is coming. 

"We will not back down. And if you feel threatened by that, then I have some very bad news for you. We will not be silenced because we are the change. And change is coming," she said.