A local politician with the far-right Alternative for Germany (AfD) was stabbed in the western German city Mannheim late on Tuesday evening, dpa learned from security sources.
The man, a local election candidate, was injured in the incident that occurred at around 10:45 pm (2045 GMT). The perpetrator was detained, dpa learned on Wednesday.
The Mannheim police only confirmed that there had been a police operation, but said they would provide information in the morning.
According to the AfD district association, the incident occurred near the market square in the Rheinau area.
The candidate only suffered cuts but he is still in hospital. He was stabbed after confronting someone who was stealing an electoral poster, the AfD association said.
The party said three people were involved in the incident and two escaped.
The far-right party said the attack was committed by left-wing extremists, information that has not been officially confirmed.
"We are shocked and dismayed," said Markus Frohnmaier, the AfD state chairman.
The incident comes shortly after the death of a German police officer, days after he was stabbed at an anti-Islam gathering in the same city last week.
Politicians are busy campaigning nationwide ahead of elections to the European Parliament that take place on June 6-9.