The German far left's star politician Sahra Wagenknecht said Sunday she was quitting the leadership of a new populist movement that aims to address the concerns of the poor and win back working-class voters but has struggled to gain momentum.

Wagenknecht, a leading member of the far-left Die Linke party, said the "Stand Up" alliance she co-founded last September needed a complete reorganisation at the top.

"The party politicians should take a step back, that also applies to myself," she told the Frankfurter Allgemeine daily in an interview.

"Their experience was needed at the start but now it's right to hand over responsibility" to grassroots supporters, she said.

The surprise retreat comes as the movement "has gone quiet", Der Spiegel weekly noted, after an initial burst of activity when it attracted 100,000 members in the first month of existence.

Some six months later that number has climbed to just 170,000, despite the fact that membership is free and requires only an online registration.

Der Spiegel said the envisaged loose alliance of leftist groups has run into scepticism from established parties and lacks a clear profile -- with even Wagenknecht's own Die Linke "unsure how to deal with the political initiative".

"Wagenknecht doesn't want to stand up anymore. It's now up to others to do the work. How embarrassing," tweeted MP Johannes Kahrs from the centre-left Social Democrats, junior partners in Chancellor Angela Merkel's coalition government.

Lawmaker Sebastian Steineke from Merkel's centre-right CDU was equally scathing.

"'Stand Up' is over already. As expected, the leftist dream remains a mere utopia," he wrote on Twitter.

Tough on migration

The "Stand Up" (Aufstehen) movement is the brainchild of Wagenknecht and her husband Oskar Lafontaine, a firebrand socialist, ex-finance minister and defector from the Social Democrats (SPD).

Its declared goal is to counter the "neoliberal policies" of Merkel's centrist coalition government and fight for secure jobs and pensions, environmental protection and "a true democracy not ruled by banks, corporations and lobbyists".

But in a controversial break with the left's "open borders" policy, the movement also advocates a tougher stance on immigration in a bid to woo back citizens who have drifted to the far right.

In the Sunday newspaper interview, Wagenknecht lamented the "bunker mentality" of Germany's leftist parties that had prevented them from embracing her project.

While the ecologist Greens and the centre-left SPD do govern with the far left on a regional level, they have never teamed up in a national coalition -- in large part because of Die Linke's uncompromising hard-left positions, such as wanting to abolish NATO.

"The (leftist) party leaderships... apparently feel so comfortable in their dead-end streets that they are missing the chance that 'Stand Up' represents," Wagenknecht said.

She later took to Facebook to reassure supporters she wasn't ditching "Stand Up" and would continue to play an active role in the movement.

"Of course I will keep working on the success of our shared project," she wrote.