About 100,000 people marched in Glasgow to demand more action on the climate crisis, organisers have said.
The protest was the biggest so far during the COP26 summit and took place alongside hundreds of similar events around the world.
Greta Thunberg joined the march but did not speak, leaving activists such as Vanessa Nakate to address a rally.
Police arrested 21 scientists who chained themselves together and blocked a road bridge over the River Clyde.
Officers also contained a group of socialist activists after pyrotechnics were set off during the march - one person was then arrested.
However the force said the day passed "largely without incident".
The "Global Day of Action for Climate Justice" march started at Kelvingrove Park in the west of the city and Queen's Park in the south at about midday and made its way along a pre-agreed three-mile route to Glasgow Green.
About 100 climate change demonstrations were held in other parts of the UK while events were also taking place in a further 100 countries including Kenya, Turkey, France, Brazil, Australia and Canada.
In London, protesters marched from the Bank of England to Trafalgar Square while another large demonstration happened in Cardiff.
The opening speeches at the protest rally at Glasgow Green came from representatives of indigenous people around the globe.
Ugandan activist Vanessa Nakate later told protesters: "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."
It was understood that Greta Thunberg decided to give space to other speakers as she had already addressed youth activists in a march and rally on Friday.
Away from the march, 21 protesters from Scientist Rebellion were arrested after chaining themselves together on the King George V Bridge in Glasgow city centre.
Charlie Gardner, an associate senior lecturer at Durell Institute of Conservation and Ecology, said scientists could not "rely on our leaders to save us anymore" and had a "moral duty to act".
He tweeted: "Over 15,000 scientists declared that we're in a climate emergency, but most aren't acting as if it's an emergency.
"We're taking this action to encourage others, scientists and all people, to rise up in rebellion against the system that is killing everything."
Police Scotland closed the bridge to pedestrians and vehicles during the protest.
A spokesperson said: "We have facilitated a peaceful protest, but to balance right to protest with public safety and rights of the wider community, our protest removal team is safely removing protesters."
After the event at Glasgow Green dispersed, Assistant Chief Constable Gary Ritchie said the atmosphere had been "generally good natured", with people in "high spirits".
He confirmed one person had been arrested after a socialist group was contained and a further 21 were arrested following the King George V Bridge incident.
He said: "Glasgow has today hosted a public protest the size and scale of which was beyond anything many of us - both within and outwith policing - can ever remember. I am pleased to say today has passed largely without incident.
"I want to thank the vast majority of those who attended both the Youth March and the Climate March for their positive attitudes and for following instructions from our officers."
On the scene - Paul O'Hare, BBC News website
Despite the grim weather there was a carnival atmosphere at the start of the march in Kelvingrove Park, with one group playing steel drums to entertain the crowd.
Flags and banners were blowing in the wind and most activists were sporting multiple layers in a bid to combat the cold and rain.
One enterprising local vendor who was selling whistles at the youth march on Friday was today offering ponchos for £3.
There was a heavy police presence and officers paid particular attention to a group of socialist activists dressed in black and wearing red face masks.
Members of the group were still being closely monitored more than five hours later when they were among the last to arrive at Glasgow Green.
A helicopter hovered overhead as the demonstration snaked through the west end and city centre.
One group set off a red flare in Finnieston but the march was good natured throughout and I saw no signs of any trouble.
The event was the largest protest in Glasgow since the Stop the Iraq War march in 2003 and groups representing a broad spectrum of causes, ranging from indigenous people to refugees, made for a colourful spectacle.
They were entertained along the way by acts including a bagpiper and a man singing karaoke dressed as Darth Vadar.
One of the highlights was a spontaneous outbreak of applause which greeted the marchers as they arrived in George Square.
It was summed by a banner which read: "Glasgow Welcomes Climate Activists".
The sun had come out by the time the mass protest arrived at Glasgow Green and some revellers even enjoyed a dance party outside the High Court in Glasgow.
But the huge crowd was later left disappointed when it emerged celebrated climate activist Greta Thunberg would not be addressing the rally.
World leaders and representatives who came to Glasgow for the climate change summit have so far made promises to curb deforestation, phase out coal, end funding for fossil fuels abroad and cut methane emissions.
But there is still a significant gap between the measures countries have committed to and what is needed to avoid more than 1.5C of warming, beyond which the worst floods, droughts, storms and rising seas of climate change will be felt.
Countries are under pressure to agree on actions for the next decade, finance loans for developing countries to cope with the crisis and finalise the last parts of how the global Paris Agreement on climate change will work.
Asad Rehman, a spokesperson for the COP Coalition, said: "We are taking to the streets across the world this weekend to push governments from climate inaction to climate justice.
"This has been the least accessible climate summit ever - with so many people side-lined at the talks or not able to make it in the first place. Today those people are having their voices heard."
Representatives of indigenous communities from North and South America joined the march in Glasgow.
Some have no formal representation at COP26, despite living at the frontline of environmental destruction caused by mining or deforestation, because they are not recognised by their governments.
Marchers came from across the UK to take part in the Glasgow march, including surfers from Cornwall.
Edinburgh COP Coalition put on 13 coaches to travel to the event while a series of "Pedal on COP26" bike rides were organised from more than 20 areas of Scotland, the longest rides being from Inverness, Aberdeen and Dumfries setting off on Friday.
The cyclists brought up the rear of the march, which took near two hours to leave Kelvingrove Park, making the line of protesters almost two miles (3.2km) long.
Police said they blocked a section of the crowd between Holland Street and St Vincent Street because of "congestion issues".
"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," a Police Scotland tweet said.
Bleak weather with heavy showers and gusts of wind greeted early arrivals to the march, but the weather improved over the afternoon.
High winds, however, continued to cause a problem, hampering efforts to set up the main stage at Glasgow Green. and speakers instead used the stage company's lorry as a platform.
Saturday's march came after many young activists - including striking school pupils - took part in a mass demonstration in the city on Friday.
The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost 200 countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives.