Prime Minister Boris Johnson has welcomed world leaders to Glasgow on the first full day of the COP26 summit. The heads of 120 countries are present, including US President Joe Biden and French President Emmanuel Macron. For Greta, however, this isn't enough.
Ms Thunberg was one of the environmental activists speaking at the "Fridays for Future" meeting of young activists at Festival Park, near the COP26 campus.
She said: "This COP26 is so far just like the previous COPs and that has led us nowhere. They have led us nowhere."
"Inside COP there are just politicians and people in power pretending to take our future seriously, pretending to take the present seriously of the people who are being affected already today by the climate crisis.
"Change is not going to come from inside there. That is not leadership - this is leadership."
The Swedish teenager led cries of "climate justice" and "no more blah, blah, blah. We're sick and tired of it and we're going to make the change whether they like it or not," she added.
Lang Banks, the director of WWF Scotland, said that he agrees with Thunberg that politicians were not fulfilling their pledges, such as the promise of $100bn to help poorer nations cut emissions by 2020.
He said: "The world leaders have not stepped up to the plate. They have an opportunity now to turn their warm words we've heard today into action.
"We cannot allow them to leave Glasgow without making sure that we are somehow closer to making sure the planet does not burn, and does not warm above 1.5C."
These aren't the only people protesting. Oxfam organised a Scottish pipe band protest, with participants dressing up as 10 leaders from the world's "highest-emitting" countries.
World leaders (including Trudeau) were all caricatured in the band which Oxfam said was guaranteed to produce a "lot of hot air".