Thousands of young climate activists massed in Berlin on Friday to press the likely successors to outgoing chancellor Angela Merkel's government to do more to slash dangerous emissions.

The Fridays for Future youth movement called the march from the Brandenburg Gate landmark through the government quarter a month after the German general election.

Holding signs such as "Make Love, Not CO2" and "The Snow Must Go On", the crowd chanted "Our future -- off the negotiating table!"

The centre-left Social Democrats are working toward a coalition with the smaller ecologist Greens party and the pro-business Free Democrats (FDP), with the aim of installing a new government by early December.

Last week the parties unveiled a preliminary roadmap for a four-year term including a planned zero-emissions target by 2045 and accelerated exit from coal energy by 2030.

But the head of the German chapter of Fridays for Future, Luisa Neubauer, has blasted the programme as "insufficient" to meet Berlin's international emission-reduction commitments, after years of what she called disappointing action by the Merkel government.

'My future? SOS' reads a placard held up by a very young activist at the climate rally in Berlin
'My future? SOS' reads a placard held up by a very young activist at the climate rally in Berlin AFP / INA FASSBENDER

The movement wants to make the 2030 coal exit target legally binding and the powerful German auto industry to stop building vehicles whose engines burn fossil fuels by 2025.

Demonstrators on Friday were particularly frustrated that the Free Democrats torpedoed a proposal by the Greens to introduce a nationwide speed limit on the famed autobahns, and booed a mention of FDP leader Christian Lindner.

"It's something that would hardly cost anything and would make a big difference," said Larena Dix, 23, an architecture student in Dresden who travelled to the capital for the rally.

Celine Schneider, also 23 and studying law, said she hoped the Greens would fight harder in the coalition talks, particularly for changes to agriculture policy to cut meat consumption.

"They shouldn't just roll over," she said of the party which rose from the environmentalist protests of the 1970s.

Biologist Joy Hatzidakis, 47, brought her three young children to the demonstration, saying their health and well-being were on the line.

"A lot of people want change but aren't doing anything -- at least we're raising our voices," she said.