A local politician with the far-right Alternative for Germany (AfD) was stabbed in the western German city Mannheim late on Tuesday evening, officials said.
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 25-year-old suspect has been taken to a psychiatric hospital, according to investigators. The police and public prosecutor's office said on Wednesday that there were clear indications he was suffering from a mental illness at the time of his arrest.
"According to the current state of the investigation, there are no concrete indications that the suspect realised that the injured party was an AfD politician when he carried out the attack," police said.
According to the AfD district association, the incident occurred near the market square in the Rheinau area. The politician suffered injuries to his ear and stomach, the AfD group in the local council said.
The victim had earlier attended an event organized by the AfD district association, according to Jörg Finkler, of the AfD parliamentary group in the Mannheim municipal council.
He then saw several people removing AfD election posters and confronted them. The party said earlier that three people had been involved in the incident and two escaped.
Finkler said he and the man are both standing for the council and are friends. He stayed with him and his wife in hospital until the early hours of the morning.
A nearby police patrol caught the suspect in a very short time after a member of the public called the emergency services, he said.
The candidate remains in hospital but is likely to be released around midday, Finkler said.
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.
There has been widespread outrage and dismay in Germany after a series of violent attacks on politicians campaigning, affecting parties across the political spectrum in the past few months.