More than two weeks later, some look pale and emaciated. One collapsed on Tuesday. Another broke down in tears as medics performed a daily check of their weight and blood pressure.

But they have still not achieved their chief objective -- a meeting with the three main candidates vying to replace Angela Merkel as chancellor when Germany goes to the polls on September 26.

"The climate crisis kills. We are on hunger strike for an unlimited period of time," a banner strewn across one of the tents proclaims in large red letters.

The activists want to meet conservative Armin Laschet, Social Democrat Olaf Scholz and the Greens' Annalena Baerbock.

All three parties have made climate policy a key issue in their campaigning, and the Greens have even pledged to make climate neutrality the top priority of the next government.

But the activists say it's not enough. For Jacob Heinze, none of the major parties is prepared "to take the necessary measures to protect us, the younger generation, from the catastrophe" that is unfolding.

- 'Time bomb' -

The activists also want the next German government to set up a committee of citizens representing the whole spectrum of society to develop measures to protect the environment.

The hunger strike is a "last resort... in the face of the extreme seriousness of our situation", the 27-year-old told AFP, long hair tied back from his gaunt face.

Just hours later, he was taken to hospital after collapsing.

Hannah Luebbert, a 20-year-old activist who is part of the support team, told AFP he was released later Tuesday and he immediately returned to the hunger strike.

Asked about the man's wellbeing earlier, Merkel's spokesman Steffen Seibert wished him a "quick recovery", but warned against drastic action as part of the demonstration.

"Hunger strikes for the climate or for a meeting with politicians can endanger your health. Everyone needs to be aware of that," he told reporters.

"Every proposal, every approach in the political debate about climate protection is welcome, but please without endangering yourself."

Luebbert said the urgency of the issues required more than conventional marches or sit-ins.

"We are sitting on a time bomb," she said. "If we don't change things quickly, in a few years it will be too late."

For evidence of this, according to the activists, you only have to look at the deadly floods that swept through western Germany in July, which experts have directly linked to climate change.

Global warming will also bring famine, they say, hence the idea of voluntary starvation.

"Food security is not something we can take for granted. We are heading for wars over the distribution of food, water and land," said Heinze.

The school and university students aged between 18 and 27 from all over Germany believe they belong to "the last generation" that can still take action.

- 'Grim and hard' -

After that, they say, scientific research has shown that the dramatic consequences of global warming will become irreversible.

For them, civil disobedience movements like Extinction Rebellion and Fridays for Future do not go far enough. Some have already carried out drastic stunts such as scaling political buildings or chaining themselves to the streets to block traffic.

"But we have seen that these different forms of action have not led to any change" at the political level, Luebbert said.

On the 15th day of their strike, the activists decided to up the ante by giving up the vitamin drinks they had been taking.

"I think we're noticing the aftermath and next week is going to be really grim and hard," said Henning Jeschke, an activist who has posted several videos of the action on Twitter.

The only response they have had so far is a phone call from Baerbock. "But even with the Greens we will not meet the climate targets we have to meet," said Luebbert.