Angela Merkel's election rival Peer Steinbrueck rallied supporters at a Berlin landmark Thursday to swing behind his bid to usher in a more socially fair Germany by toppling the chancellor.

"In three days, it's in your hands to bring about a change of government in Germany," the 66-year-old challenger in Sunday's elections told the crowd in the famous Alexanderplatz, which once marked East Berlin's centre.

"Freedom, justice and solidarity," he said, stressing the values his Social Democratic Party (SPD) had represented for 150 years and would continue to fight for, as he critically picked off Merkel's policies.

Under Berlin's iconic Television Tower, Steinbrueck, donning a red tie in his party's signature colour, constantly moved and turned on the podium at the centre of the open-sided tent in a lively address to the mixed gathering of several thousand.

More crowded around the outside, some enjoyed an after-work beer on a fresh but clear evening, while others broke off from shopping to crane for a view of several SPD leaders who joined the homestretch push for votes.

"He is completely different from Mrs Merkel and that's good," Daniel Boese, 39, a lecturer from Berlin, who has been an SPD member for more than 20 years, told AFP.

"Mrs Merkel talks a lot and does nothing."

After a gaffe-prone campaign, the self-styled straight-talking Steinbrueck has struggled to chip away at Merkel's popularity, with the SPD trailing the conservatives by a 10-point margin in an Insa poll Thursday.

A surly middle-finger front-page photo of him last week followed other missteps, such as grumbling the chancellor's salary is too low, suggesting that Merkel owes her success to being a woman and dubbing two Italian election winners "clowns".

Brigitte Ruehl, 71, from Berlin said she was still undecided about who to vote for but was concerned in particular about former East Germans' pensions being lower than their Western counterparts.

"After 23 years of unity, it is time that they were aligned," she complained, adding she had voted for Merkel's conservatives and the Greens under Germany's double vote system in 2009.

Asked if she was disappointed in Merkel, she shook her head, paused, and then added however that the chancellor was "a bit cautious."

Steinbrueck, a former minister, is calling for a national minimum wage while Merkel favours more flexible pay agreements, and targets a yawning wealth gap and Germany's large low-wage sector.

Pensions, care reform, solidarity with European partners and tax cheating all featured in the SPD candidate's emphatic call for voters to turn out and cast their ballots, in which he also blasted Merkel for empty words and a "wait and see" approach.

Her policy phrases are "all stickers on empty bottles," he said, to enthusiastic applause by supporters, some of whom held high big pictures of him with the slogan "Plain Talking".

Merkel's success in retaining power with her current centre-right coalition hinges on the precarious fortunes of her pro-business allies, the Free Democrats.

But the electoral maths may force her into a left-right "grand coalition" with her rivals instead.

Steinbrueck has ruled himself out of such a tie-up with Merkel despite having been her finance minister under a 2005-2009 grand coalition, after which the SPD went on to score its worst ever result, at 23 percent.

But Oskar Niedermayer, a political scientist at Berlin's Free University, said in the 2009 vote, a third of the electorate had said they only made up their minds whom to vote for in the last few days.

"I reckon on it also being the case now," he told AFP.

Stefanie Elies, 48, an SPD member who has rallied door-to-door during the campaign, said she needed no convincing.

"No, I'm not disappointed (in Steinbrueck)," she said.

"I was realistic in my expectations. We knew him for quite a long time as a minister, and as an outspoken politician."