Labour leader Jeremy Corbyn thanked everyone who showed up at the central London demonstration for their very public display of "determination to combat climate change and to protect our environment and our planet".
He said: "An unequal and unjust world is something we have to deal with.
It is not just us here, it is how we act together to bring about climate sustainability." To cheers, he told the youngsters in the crowd that "you and a whole generation have brought the issue centre stage and I am absolutely delighted about that".
In condemning US President Donald Trump's actions on climate change, he said: "What is disgraceful is to have the president of a major country like the United States saying he will walk away from the Paris climate change accord.
"Let's have no more if this hand-holding of Donald Trump.
Let's simply say that we want every country on board on this, every country fully signed up to Paris and going a lot further than that."