Assailants wielding wooden bars beat unconscious a lawmaker for the far-right Alternative for Germany (AfD) party in the northern city of Bremen in what police described as a politically-motivated attack.
Bremen police, who have begun an investigation into the assault, said two craftsmen called an ambulance after discovering Magnitz lying on the ground.
Frank, 66, was rushed to hospital.
The AfD say they keep track of attacks against their officers, members, and their homes, and gave statistics to Breitbart London which showed that attacks have risen from only four in 2014 to well over 600 in 2017.
The reported attack came just three days after an unknown substance exploded outside the AfD office in the eastern town of Doebeln, in Saxony state.
The party published a photo of Magnitz unconscious on a hospital bed, his face bleeding and swollen with a gash on his forehead.
Magnitz was in hospital, and he told the dpa news agency that doctors want him to remain there until the weekend.
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He added that while he had received threats, he had not considered any of them concrete.
Magnitz is associated with the extreme right of the party, including its firebrand leader in the eastern state of Thuringia, Bjoern Hoecke.
Foreign Minister Heiko Maas, of the center-left Social Democratic Party, tweeted: "Violence should never be a means for political disputes-totally regardless against whom or what the motives for it are".
Former Green Party co-chair Cem Özdemir expressed hope that the perpetrators will be punished. "Anyone who fights hatred with hatred always lets hatred win in the end", he tweeted.
"Today is a black day for democracy in Germany", the AfD said, noting that Magnitz remains in serious condition. It entered the national parliament in 2017 and is the biggest opposition party there.
AfD's party leaders Alexander Gauland and Alice Weidel called Magnitz's assault a "result of the incitement to hatred by politicians and media against us".
The government and politicians across the spectrum condemned the attack.
In 2015, a leading candidate for mayor in Cologne was stabbed in the neck by a far-right extremist.