Speaking at a rally on Friday in front of the Reichstag parliament building, Thunberg told cheering supporters they needed to keep up the pressure on Germany's political leaders past election day.

"It is clearer than ever that no political party is doing close to enough. But it's even worse than that. Not even their proposed commitments are close to being in line with what would be needed to fulfil the Paris Agreement" on curbing climate change, she said.

"Yes, we must vote, you must vote, but remember that voting only will not be enough. We must keep going into the streets."

As Germany's top parties were set to hold final rallies, the Fridays for Future youth movement said it was holding strikes in more than 400 German cities and towns.

At the largest event in Berlin, Thunberg said the political class was failing younger generations.

"We need to become climate activists and demand real change because remember: change is now not only possible, it is also urgently necessary," she said.

"When enough people demand change, then change will come on election day."

The race has boiled down to a two-way contest between Social Democrat Olaf Scholz, the centrist finance minister, and Armin Laschet from Merkel's conservative Christian Democrats. They were set to address crowds later Friday, in Cologne and Munich respectively.

Polls give Scholz a small lead with about 26 percent over Laschet at 22 percent. The candidate from the ecologist Greens, Annalena Baerbock, trails in the mid-teens.

Thunberg stopped short of endorsing the Green party, whose programme Fridays for Future activists argue falls short of what is needed to effectively curb global warming.