Tens of thousands of marchers took to the streets of Brussels on Sunday to mark the start of the UN climate summit in Poland.

Organisers said the event was the biggest climate march ever in Belgium, with 65,000 protesters braving steady drizzle, according to police.

The well-mannered crowd was filled with activists and many families, with children holding placards asking politicians to fight harder to assure a greener future.

Speaking through a blow-horn, activist Evert Nicolai of Oxfam Action urged leaders to do what was needed towards fighting climate change.

"Politicians where are you? You have to be with us, the people. There is no planet B," Nicolai said.

The march, dubbed Claim The Climate, ended with speeches and performances at the Parc Cinquantenaire that overlooks the European Union institutions.

Public transport throughout the city was free on Sunday for the occasion and the SNCB national train service offered a cut-rate ticket of five euros.

Trains from other Belgian cities were packed with protesters, with many complaints on social media that the SNCB had not adequately anticipated the surge in travellers.

The protest was timed for the launch of the COP24, the UN climate summit where officials from 200 nations will aim to boost efforts to avert runaway climate change.

Greenpeace dozen denied bail over Slovak coal mine demo
Bratislava (AFP) Dec 2, 2018 –

A court on Sunday denied bail to 12 Greenpeace activists who staged an anti-coal protest at Slovakia's largest and oldest brown coal mine, a move that the global environmental group called a "disgrace".

The activists were detained after hoisting a banner saying "End coal age!" on the tower of a lignite mine in Novaky, central Slovakia, on Wednesday.

"The judge took 12 defendants into preventive custody. The reason is the fear that they will continue to commit crimes," Roman Tarabus, a Prievidza county court spokesman said in a statement on Sunday.

The court ruling coincides with launch of the UN's COP24 climate conference in neighbouring Poland, amid urgent warnings by scientist about the rapid pace of global warming.

The activists — from Belgium, Czech Republic, Finland, Germany and Slovakia — are accused of criminal acts and face up to five years in prison if found guilty.

The court described their crime as "damaging and compromising the operation of a device of public benefit," Tarabus said, adding that the "protest stopped quarrying and extraction of coal for five hours."

Greenpeace says the activists will be held in detention until their trial "which could take months". The organisation has launched an appeal.

"These 12 activists took peaceful action to protest against the dirtiest fossil fuel and their continued detention is a disgrace," Jennifer Morgan, executive director of Greenpeace International told AFP in an emailed statement.

"We call on Slovakia for their immediate release," she added.