A far-right German MP was seriously injured in a "politically motivated" gang attack in the north of the country, police said Tuesday.

Frank Magnitz, the leader in Bremen of the anti-immigration populist party Alternative for Germany (AfD), was assaulted in the city center on Monday afternoon.

"Given the victim's work, we believe that this is a politically motivated act," police said.

The AfD party published a photo of Magnitz unconscious on a hospital bed, his face bleeding and swollen with a gash on his forehead.

It said three people with their faces covered had carried out the attack.

"They hit him with a piece of wood until he was unconscious and then kicked him on the ground," a statement from the party said, adding that a construction worker had intervened to stop the assault.

AfD leader Joerg Meuthen said Magnitz was "beaten almost to death."

Last week, an explosive device detonated in a rubbish bin damaged an AfD office in Saxony.

The AfD entered Germany's parliament with almost 13 percent of the vote last September.