She criticised world leaders for what she called a lack of action, saying Cop26 was a failure and was “now a global greenwashing festival".
Some activists have been underwhelmed by the commitments made so far, many of which are voluntary, exclude the biggest polluters or set deadlines decades away.
“Out here, we speak the truth. The people of power are obviously scared of the truth. Yet no matter how hard they try, they cannot escape from it,” Ms Thunberg told the rally.
“They cannot ignore the scientific consensus and above all, they cannot ignore us, the people, including their own children.
“They cannot ignore our screams as we reclaim our power. We are tired of their blah, blah, blah. Our leaders are not leading. This is what leadership looks like.”
Following a march of thousands of protesters from Kelvingrove Park to George Square, passing the Cop26 venue at the SEC on the way, Ms Thunberg said world leaders are “fighting to maintain business as usual”.
An estimated 8,000 people were expected to take part in the march through Glasgow, organised by Fridays For Future, despite UK government ministers saying that young people missing school to attend the demonstration could face fines.
At the summit, some 40 countries have agreed to phase out coal power over the coming decades — but the biggest coal consumers — China, India and the US — have not signed up.
But in one of the first major announcements of Cop26, more than 100 leaders did back a pledge to reverse deforestation.
Ms Thunberg said there needed to be “immediate, drastic, annual emission cuts unlike anything the world has ever seen”.
“The question we must now ask ourselves is, what is it that we are fighting for? Are we fighting to save ourselves and the living planet? Or are we fighting to maintain business as usual? Only to say that we can have both,” she said.
“But the harsh truth is that is not possible in practice. The people in power can continue to live in their bubble filled with their fantasies, like eternal growth on a finite planet and technological solutions that will suddenly appear seemingly out of nowhere and will erase all of these crises just like that.
“All this while the world is literally burning, on fire, and while the people living on the front lines are still bearing the brunt of the climate crisis.”