Environmental groups, charities, trade unionists and indigenous people joined the protest, which began in Kelvingrove Park around noon before making its way through the city centre.
Climate activist Greta Thunberg was due to address the rally in Glasgow Green but according to reports, she cancelled her appearance to let others take the stage.
Crowds were addressed by fellow activist Vanessa Nakate and leaders of indigenous groups, among others.
Ms Nakate, from Uganda, said: "The climate and ecological crises are already here. But so are citizens from around the globe.
"Leaders rarely have the courage to lead. It takes citizens, people like you and me, to rise up and demand action. And when we do that in great enough numbers, our leaders will move."
Ms Thunberg addressed youth activists during a march and rally on Friday.
People travelled from across the country to join the action today, which comes in the middle of the COP26 negotiations in the city.
Jason Cook, 54, from Wootton Bassett, Wiltshire, said he and two friends were marching because they were tired of hearing "blah, blah, blah" from leaders on climate action, echoing the criticism of Cop26 and world leaders by Ms Thunberg.
He said: We don't want to hear any more blah, blah, blah."
The three men wore helmets, each adorned with a sign which said "blah".
Dave Knight, 51, from Wiltshire, added there had been "lots of words, but we really need action - the end of fossil fuels as soon as possible".
As the march entered Glasgow city centre a group of activists dressed as rats stood across the road holding smoke flares.
The group held up signs, which when read together, said: "When Cop fails, buy walls, buy guns, buy bombs.
"Bye bye climate migrants - profit over planet."
Meanwhile, on Holland Street, a group of protesters were kettled by police with officers eventually removing the group.
A Police Scotland statement said: “Following an escalation in their conduct a small number of people from the group who deliberately stopped on the parade route at the junction of Holland Street and St Vincent Street were contained by police on the grounds of public safety.”
Climate scientist activists from the protest group Scientist Rebellion chained themselves together across King George V Bridge, one of the key routes through the city centre, and were later taken away by officers.
Trains into Glasgow were packed with protesters this morning despite the weather, and 13 coach loads of supporters arrived from Edinburgh.
Police have yet to confirm figures but up to 100,000 were expected to gather in Glasgow today, with eye witnesses suggesting more had gathered in Glasgow Green.
First Minister Nicola Sturgeon urged people to attend the protest and exercise their right to peaceful protest.
Millions more are due to take part in at least 20s other countries across the world as part of a mass mobilisation organised by the COP26 Coalition, a UK-based civil society partnership campaigning for climate justice.
Demonstrations are also taking place simultaneously in at least 12 other cities across the UK.
Demands include ending extraction and use of fossil fuels, rejecting “false solutions” such as future technology designed to extract carbon dioxide from the atmosphere, climate justice and financial support for developing countries to cut emissions.
Numerous side demonstrations are also being staged.