Thunberg said pledges from some nations made during COP26 to accelerate their emissions cuts amounted to little more than “a two-week long celebration of business as usual and blah, blah, blah”.
“It is not a secret that COP26 is a failure,” she told the thousands of people at the protest.
“This is no longer a climate conference. This is now a global greenwashing festival.”
Delegates from nearly 200 countries are in Glasgow to hammer out how to meet the Paris Agreement goals of limiting temperature rises to between 1.5 and 2 degrees Celsius.
The first week of talks saw countries announce plans to phase out coal use and to end foreign fossil fuel funding, but there were few details on how they plan the mass decarbonisation scientists say is needed.
The promises followed a major assessment that showed global CO2 emissions are set to rebound in 2021 to pre-pandemic levels.
“They cannot ignore the scientific consensus and they cannot ignore us,” said Thunberg.
“Our leaders are not leading. This is what leadership looks like,” she said gesturing to the crowd.
Two days of demonstrations are planned by activist groups to highlight the disconnect between the glacial pace of emissions reductions and the climate emergency already swamping countries across the world.
Onlookers to Friday’s march lined the streets and hung out of windows to watch the stream of protestors, who held banners reading “No Planet B” and “Climate Action Now”.
“I’m here because the world leaders are deciding the fate of our future and the present of people that have already been impacted by climate crisis,” said 18-year-old Valentina Ruas.
“We won’t accept anything that isn’t real climate policy centred on climate justice.”
Students were out in force, with some schools allowing pupils to skip lessons to see the march and one young green warrior holding a placard that read: “Climate change is worse than homework”.
Experts say a commitment made during the high-level leaders’ summit at the start of COP26 by more than 100 nations to cut methane emissions by at least 30 percent this decade will have a real short-term impact on global heating.
But environmental groups pointed out that governments, particularly wealthy polluters, have a habit of failing to live up to their promises.
Vanessa Nakate told the crowd 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.”
Countries came into COP26 with national climate plans that, when brought together, put Earth on course to warm 2.7C this century, according to the UN.
With just 1.1C of warming so far, communities across the world are already facing ever more intense fire and drought, displacement and economic ruin wrought by the Earth’s heating climate.
“Scientists have done what they need to do, they’ve told us about the problem. Young people have done what they need to do by calling attention to this issue,” said Natalie Tariro Chido Mangondo, a Zimbabwean climate and gender advocate.
“And it’s just up to our leaders to get their act together.”
Campaigners say they expect up to 50,000 demonstrators in the Scottish city on Saturday as part of a global round of climate protests.
Summit organisers on Thursday confirmed a number of positive Covid-19 tests among attendees, but have said they will not provide figures on how many.