A far-right German MP has been seriously injured in a “politically motivated” attack in Bremen, according to police.

Frank Magnitz, a politician in Germany’s national parliament who leads the local branch of the far-right Alternative für Deutschland, was attacked on Monday evening near a theatre in the city. Police have urged witnesses to come forward.

AfD said early on Tuesday Magnitz was ambushed by three masked men after he left a local newspaper’s new year’s reception. It said he was beaten unconscious with a piece of wood and then kicked in the head as he lay on the ground. He is being treated in hospital.

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

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

The foreign minister, Heiko Maas, a centre-left politician who has been a strong critic of AfD, tweeted that “violence must never be a means of political confrontation no matter against whom or what the motives are”.

He said “there is no justification for this”, and called for those responsible to be punished.

Other politicians from established parties agreed, including the prominent Green party politician Cem Özdemir. He 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 is represented in all of Germany’s 16 state parliaments. It entered the national parliament in 2017 and is its biggest opposition party.

The relationship between AfD and the country’s established political parties is characterised by mutual contempt. “The cowardly and life-threatening attack against Frank Magnitz is the result of constant agitation against us by politicians and media,” the party co-leaders, Alexander Gauland and Jörg Meuthen said in a statement.

AfD took 10% of the vote in Bremen in the 2017 national election, below its nationwide result of 12.6%, and the region is not considered a party stronghold.