About 3,000 students gathered on Berlin's Potsdamer Platz square, with smaller demonstrations held across Germany as part of a nationwide "school strike".

"I don't see why anyone should have to go to the front lines for politicians," Alex Krzeszka, a 15-year-old student, told AFP at the Berlin rally.

"I don't see it as morally right, and I think war should never be the solution. Problems should be solved diplomatically."

Germany, like other European countries, has sought to build up its armed forces in response to Russia's 2022 full-scale invasion of Ukraine and the threat of further aggression against NATO members.

Chancellor Friedrich Merz has vowed to turn the Bundeswehr into Europe's largest conventional army, banking initially on a voluntary recruitment drive.

The government this year started requiring all 18-year-old men to fill out questionnaires about their interest and fitness for short-term military service.

Women are also being asked to fill out the forms, but cannot be compelled to do so under current German law.

Among the signs being waved by protesters in Berlin was a poster that read "We are not cannon fodder" while another demanded: "Send Friedrich Merz to the front line!"

For now at least, German lawmakers have decided against bringing back mandatory conscription, which Germany suspended in 2011.

But some politicians have expressed doubts about whether ambitious recruiting targets can be achieved without some from of conscription.

Plans call for strengthening the Bundeswehr from about 185,000 active-duty troops now to to 260,000 by 2030, while roughly quadrupling the size of the reserves to 200,000.

The Bundeswehr shrank dramatically after the end of the Cold War as countries across Europe slashed defence budgets.

In the 1980s, West Germany alone had fielded a military of nearly 500,000 troops.

"I think they should definitely advertise for the Bundeswehr, but it absolutely shouldn't be compulsory," Leander Martinez, a 16-year-old student from Berlin, told AFP.

"Reintroducing conscription is nothing other than rearmament," Leon Reinemann, a student who helped organise the school strike in the western city of Koblenz, told broadcaster NTV.

He defended the fact students were skipping classes, saying that "a single day of absence from school is significantly less serious than six months in the barracks".

Others took a more staunchly pacifist stance at the Berlin demonstration.

"I'm against conscription and against war propaganda," Tillmann, a 19-year-old student who declined to give his last name, told AFP.

"And I think murdering someone is always wrong, even if the state says that someone should be murdered. There's nothing more important than human life."