Speaking at a rally outside the conference venue, Thunberg called for tougher rules to clamp down on polluters instead of what she termed distant, non-binding pledges.
"It is not a secret that COP26 is a failure. This is no longer a climate conference. This is now a global greenwashing festival."Swedish campaigner Greta Thunberg at a protest
She said pledges from some nations to cut down on emissions amounted to little more than "a two-week long celebration of business as usual and blah, blah, blah".
Delegates from nearly 200 countries are in Glasgow to chart out a plan to meet the Paris Agreement goals of limiting temperature rises to between 1.5 and 2 degrees Celsius. Several countries had proposed ideas to eliminate the use of coal and to end foreign fossil fuel funding.
People lined the streets in protest, as children held hand-drawn signs reading, “Stop deforestation”, “Save the planet!” and “Act now”.
It was the largest protest to take place in Glasgow since the climate talks, known as COP26, began nearly a week ago.
"They cannot ignore the scientific consensus and they cannot ignore us," Thunberg said.
Activists have been rallying for two days to discuss the disconnect between the pledges of emissions reductions and the way the leaders are addressing the climate emergency.
With just 1.1C of warming so far, countries across the world are already facing intense fire and drought, displacement and economic ruin wrought by the Earth's heating climate.
More than 100 nations have vowed to cut methane emissions by at least 30 percent this decade but activists believe it will have a real short-term impact on global heating.
Activist Vanessa Nakate said at the procession that people in her native Uganda were "being erased" by climate change.
"People are dying, children are dropping out of school, farms are being destroyed," she said. "Another world is necessary. Another world is possible."
(With inputs from AP)