A far-right German politician has been beaten unconscious by a group of men in the north-west city of Bremen in an assault that has drawn condemnation from some of his party's fiercest opponents.

Key points: Frank Magnitz's party says he was beaten with a piece of wood and then kicked in the head

Frank Magnitz's party says he was beaten with a piece of wood and then kicked in the head Alternative for Germany is currently the largest opposition party in the German Parliament

Alternative for Germany is currently the largest opposition party in the German Parliament Politicians from more established and mainstream parties have condemned the attack

Bremen police said they believe the attack on Frank Magnitz, a politician in Germany's National Parliament who leads the local branch of the Alternative for Germany (AfD), was politically motivated.

They called for witnesses to the attack near a city theatre at about 5:20pm on Monday to come forward.

Mr Magnitz was beaten over the head with an unidentified object by at least three men wearing dark clothing and hoods or hats, who then fled, police said.

Two workers who were loading a car nearby found him lying on the ground and called an ambulance.

The 66-year-old was hospitalised.

The party said Mr Magnitz was ambushed after he left a local newspaper's new year's reception, beaten unconscious with a piece of wood and then kicked in the head as he lay on the ground.

Bremen, Germany's smallest state, holds a regional election on May 26, the same day as European Parliament elections in which AfD hopes to make gains.

Frank Magnitz was hospitalised after the attack. ( dpa via AP: Michael Kappeler )

Chancellor Angela Merkel's spokesman, Steffen Seibert, wrote on Twitter "the brutal attack on lawmaker Frank Magnitz in Bremen must be strongly condemned. Hopefully police will quickly succeed in catching the perpetrators".

Foreign Minister Heiko Maas, a centre-left politician who has been a strong critic of AfD, tweeted "violence must never be a means of political confrontation — no matter against whom or what the motives are".

"There is no justification for this," he said, calling for those responsible to be punished.

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That was echoed by other politicians from established parties, including prominent Green party politician Cem Ozdemir, who said AfD must be countered by legal means, not violence.

"Anyone who fights hatred with hatred always lets hatred win in the end," he wrote on Twitter.

AfD's rapid rise

AfD is represented in all of Germany's 16 state parliaments.

It entered the national Parliament in 2017 and is currently the biggest opposition party there.

AfD views the country's established political parties with contempt, and the feeling is mutual.

"The cowardly and life-threatening attack against Frank Magnitz is the result of constant agitation against us by politicians and media," party co-leaders Alexander Gauland and Joerg Meuthen said in a statement.

Critics of the AfD have called for those responsible to be punished. ( dpa via AP: Helmut Reuter )

AfD took 10 per cent of the vote in Bremen in the 2017 national election, below its nationwide result of 12.6 per cent.

Bremen is not considered a stronghold of the six-year-old party, unlike three states in Germany's ex-communist east that hold regional votes in September and October.

Germany has seen other attacks on politicians in recent years.

In 2015, a far-right extremist stabbed in the neck a leading mayoral candidate for Cologne, who at the time was in charge of housing refugees.

Henriette Reker was elected mayor the following day while in an induced coma and took office about a month later.

In 2017, a man with a knife attacked the mayor of Altena in western Germany.

The mayor was known for voluntarily taking in more asylum seekers than the small town was obliged to.

AP