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.