Two Britons were deported and more than 900 people were arrested as tens of thousands of demonstrators took to the streets of Copenhagen in a protest over climate change, police said.
Tensions rose and there were clashes with police during the protest to demand a fair and binding new global deal to tackle climate change.
It marked the halfway point of crunch UN climate talks in the Danish capital.
The protest included a mass rally outside the country's parliament and a march to the conference centre where negotiations on a new global warming deal are taking place.
A Copenhagen police spokeswoman confirmed two Britons were deported for vandalism and spitting on a police officer during the protests. About 400 people had been arrested during the demonstrations, she said.
Official police estimates put the number of protesters at 25,000, but organisers said as many as 100,000 had joined the march from central Copenhagen, waving banners that read "Nature doesn't compromise" and "Climate Justice Now".
The protests were largely peaceful for most of the day, but the mood changed late in the afternoon when hundreds of youths dressed in black threw bricks and smashed windows of buildings in the centre of the city.
Demonstrators were forced to the ground and then bundled into vans, according to reports.
To mark The Global Day of Action on climate change, campaigners also staged events around the world, including a four-minute "flashdance" with lights performed outside the Houses of Parliament, with volunteers across London collecting messages from citizens to deliver to MPs.
Actress Helen Baxendale was among the celebrities joining the events in the Danish capital, along with model Helena Christensen and former Irish president and UN high commissioner for human rights Mary Robinson. Baxendale said thousands of people from all over the world were trying to encourage their leaders to take "firm and fair action".
