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 was a major challenge.
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 place Scholz's SPD at 25 percent and Laschet's conservatives at 22 percent, putting the gap between them well 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 government," he said.
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 and 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 told AFP at the Berlin protest.
Luisa Neubauer, 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," Neubauer told AFP.
The protest movement also hit out at Scholz, after he sent a tweet in support of Friday's marches.
"We don't want to spoil the good mood -- but we are striking against YOUR government today, Olaf," they wrote, referring to the SPD, which has been a junior coalition partner in three out of four Merkel governments.
Around 60.4 million Germans are called to the polls on Sunday and most voters have cited climate protection among their top priorities.
While younger voters are leaning Green, under-30s only make up around 15 percent of the electorate while the above-60s make up 38 percent.
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," she said.
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.
But critics have labelled the Greens a "prohibition party" that will drive up petrol, electricity and air ticket prices.
The ecologist party has advocated stopping coal energy usage by 2030 instead of the current 2038, and wants production of combustion engine cars to end from the same year.
It has slammed Merkel's right-left government of paying only lip service to environmental protection, while seeking to maintain status quo.
Hitting out against the transformation demanded by the Greens, Laschet on Friday instead appealed to the workers in Germany's vital industries.
Germany must "still have a strong automobile industry, a steel industry, a chemical industry in 20 years," he said.
"The world's climate is not served if companies relocate, they will produce elsewhere under worse social conditions, under worse environmental conditions," Laschet warned.
