Germany has told climate protesters that police will come down hard on them if they try to stop traffic at airports.
The warning comes after activists in southern Germany were handed 30 days in preventive custody without trial.
A group called “The Last Generation” is promising to widen its civil resistance to cover all areas linked to climate change.
It has not ruled out protesting on runways after Dutch activists blocked private jets at Amsterdam’s Schiphol Airport.
The Last Generation’s regular stunts have set off a debate in Germany over whether its protest tactics are legitimate.
Asked about possible airport protests, an interior ministry spokesman said Germany's federal police would get involved.
The government has “drawn a clear line where legitimate protest ends, namely where crimes are committed,” he said.
“That calls for a firm and resolute response from the police, and we are sure the police will react firmly if there are further such actions.”
The Schiphol protests led to hundreds of people being arrested after they occupied part of the tarmac used for private jets.
In Germany, police in the state of Bavaria are using a law amended last year to detain likely protesters for 30 days.
British police likewise made pre-emptive arrests on Sunday in a failed attempt to stop a motorway protest.
But Amnesty International and other activists in Germany say the 30-day detention goes too far.
Activists said they knew of no other case in modern Germany in which police had detained people for so long.
“We don’t want your sympathy, we want you all to resist on the streets,” said Jakob Beyer, one of those detained, in a defiant plea for more protests.
Protesters have frequently blocked roads to demand more radical climate policies from Chancellor Olaf Scholz’s government.
In one case, mashed potatoes were thrown at a painting by Claude Monet, in an echo of a protest involving a Vincent van Gogh artwork in London.
A particularly bitter debate ensued after blocked roads were blamed for the death of a female cyclist.
Emergency services could not reach the woman after she collided with a cement mixer in Berlin.
Some conservative politicians caused a stir by comparing the activists to the Red Army Faction, notorious West German terrorists of the 1970s.
“We have to prevent a climate RAF from developing,” former conservative minister Alexander Dobrindt said on German television on Tuesday.
Another Green MP, Matthias Gastel, said he did not agree with Last Generation's tactics but rejected the comparison.
“The warning of a new RAF is unhistorical and downplays the inhumanity that comes with terrorism,” he said.