Chancellor Angela Merkel urged Germans to elect her would-be successor for the sake of German stability, in a strong pitch for her party as candidates made their final bid for support hours ahead of Sunday's vote.
As hundreds of thousands of climate activists led by Greta Thunberg descended on streets across Germany to demand change and more protection for the environment, the outgoing Merkel acknowledged global warming as a major challenge in coming years.
But she said that protection was best achieved "not with bans and commands" but with technological progress, as she reminded voters that it mattered who led Europe's biggest power.
In a strong appeal to a predominantly older electorate, Merkel said: "To keep Germany stable, Armin Laschet must become chancellor, and the CDU and CSU must be the strongest force."
The candidate of Merkel's CDU-CSU alliance, Laschet, 60, has been trailing his Social Democrat challenger Olaf Scholz in the race for the chancellery.
But final polls put the gap between them within the margin of error, making the vote one of the most unpredictable in recent years. The Greens, polling in the mid teens, were in third place, with a clear likelihood of being part of Germany's next coalition government as a junior partner.
In the race for votes, Scholz, Germany's current finance minister, said it was time for a "fresh start for Germany" after 16 years of Merkel at the top.
"We need a change of government and we want an SPD-led goverment," he said.
- 'Not enough' -
But even the change promised by Scholz or the Greens was not enough, Thunberg told cheering Fridays for Future youth supporters outside the Reichstag parliament building, stressing that they needed to hold Germany's political leaders to account past election day.
"It is clearer than ever that no political party is doing close to enough... 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."
Organisers said the rallies had drawn 620,000 people to more than 470 cities in towns across the country.
"Climate is an important issue and if this continues things are going to get worse and worse," 14-year-old Louise Herr, who turned up at the Berlin protest, told AFP.
The head of Fridays for Future's German chapter said the country, one of the world's top emitters of greenhouse gases, had an outsize responsibility to set an example, with time running out to reverse destructive trends.
"That is why we are calling this the election of a century," Luisa Neubauer told AFP.
- 'Need a fresh start' -
All three leading parties have said they aim to implement a climate protection agenda if elected, with the Greens presenting the most ambitious package of measures.
Greens chancellor candidate Annalena Baerbock, who joined one of the Fridays for Future rallies in Cologne, told Die Welt newspaper that she hoped the protests would give her party "tailwinds" heading into the vote.
"The next government has to be a climate government -- that will only work with a strong Green party."
Their central demand is to limit the warming of the Earth to a maximum of 1.5 degrees Celsius (2.7 degrees Fahrenheit) as laid out in the 2015 Paris climate accord.
Despite Merkel's vocal support of climate protection measures, Germany has repeatedly failed to meet its emission reduction targets under the pact.
In a landmark ruling in April, Germany's constitutional court found the government's plans to curb CO2 emissions "insufficient" and placed an "unfair burden" on future generations.
Around 60.4 million Germans are called to the polls on Sunday and most voters have cited climate protection among their top priorities.