A Slovak court ordered 12 Greenpeace activists, who were detained after protesting against a coal mining company, to remain in custody on Sunday until a trial.
Slovakia's Prime Minister Peter Pellegrini, however, said the court's decision was questionable.
Fifteen activists from the Czech Republic, Belgium and Finland were detained on Wednesday after hanging a banner reading "Stop the age of coal" from a tower at a coal mine that supplies one of Slovakia's most polluting power plants.
No one was harmed during the protest but the mining company, HBP, said 342 miners underground were put in danger as all operations at the premises were halted for several hours.
Three of the activists were released on Wednesday, while the rest were charged with a criminal offence of endangering a strategic utility.
The regional court in Prievidza, central Slovakia, did not set a date for a trial. Lawyers for the activists filed an appeal against the decision to keep them in custody, Greenpeace told Reuters.
If the decision is upheld, they could stay in the pre-trial custody for up to 12 months.
"The court's decision is unprecedented. Activists are not criminals, they staged a non-violent protest against a company that has been polluting the region and Slovakia for decades," Greepeace's chief in Slovakia, Ivana Kohutkova, said in a statement.
Prime Minister Pellegrini said only perpetrators who pose a risk to society should be held in pre-trial custody.
Slovakia's economy minister announced last month that the state will phase out subsidies for coal mines supplying one of the country's most polluting power plants from 2023.
The government supports privately owned Hornonitrianske Bane Prievidza (HBP), Slovakia's only coal miner, paying around 100 million euros ($114 million) a year, which helps maintain thousands of jobs.
The company produced 1.8 million tonnes of brown coal last year, supplying the Novaky power plant in central Slovakia. The facility is operated by Slovenske Elektrarne, a utility co-owned by the state, Italy's Enel and Czech energy group EPH.
Closing the mines has long been contentious as they employ around 4,000 people directly and 11,000 indirectly, although there are potentially opportunities in Slovakia's booming car industry for retrained workers to secure another job.