A local politician with the far-right Alternative for Germany (AfD) was injured after being stabbed in the western German city Mannheim late on Tuesday evening, officials said.
Local council candidate Heinrich Koch suffered wounds to his ear and stomach but was not critically injured in the incident at around 10:45 pm (2045 GMT).
A 25-year-old suspect was detained. He fled but did not resist arrest when he was caught, said the public prosecutor's office. He used a cutter knife in the attack.
The suspect was 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.
Koch however said he thought the attack was politically motivated, after seeing people tearing down AfD posters and after calls were made online for attacks on AfD politicians in Mannheim, he told dpa.
Koch is set to be released from hospital in the course of the day.
The incident occurred near the market square in the Rheinau area, the AfD district association said earlier. Koch had attended an evening event organized by the AfD district association, according to Jörg Finkler, of the AfD parliamentary group in the Mannheim municipal council.
Koch spotted several people removing AfD election posters and confronted them. The party said three people had been involved in the incident and two escaped.
A nearby police patrol caught the suspect in a very short time after a member of the public called the emergency services, he said.
"We are shocked and dismayed," said Markus Frohnmaier, the AfD state chairman.
A video of the incident was also passed to dpa, which Koch said he filmed himself. It shows him filming running after a younger man across the market square and shouting "Stop! Stop right there!"
The younger man is carrying several AfD election posters under his arm and is holding a box cutter. The words "Get down now!" are heard then there is a scuffle. A man can be seen lunging with the knife, then the following images are blurred.
Koch said he did not initially realise the man with the posters was carrying a knife, and also did not initially notice he had been injured. "I didn't realise that in my adrenaline rush, I wanted to catch him," he said.
The far-right party said the attack was committed by left-wing extremists and the chairman of the AfD parliamentary group in Baden-Württemberg's state parliament, Anton Baron, slammed the stabbing as an expression of totalitarian power fantasies.
"Whether Islamism or left-wing extremism - anyone who neither accepts freedom of expression nor has reasonable arguments now reaches for the knife. This is shocking and shows how deeply divided our society has become."
The Mannheim incident comes shortly after the death of a German police officer, who succumbed to his injuries on Sunday, two days after he was stabbed at an anti-Islam gathering in the same city.
Lawmakers in Mannheim condemned Tuesday's attack. "This cowardly act is despicable and cannot be justified by anything. Anyone who attacks election candidates is questioning our free, equal, universal, direct and secret elections - and thus the basis of our democracy," said Lord Mayor Christian Specht.
Franziska Brantner, a member of the Bundestag from the Green Party in nearby Heidelberg, said she was horrified to learn of the attack. "The escalation of violence against politicians of all parties that we have witnessed in recent weeks is in no way justifiable," she said.
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. Social Democrat election campaigner Matthias Ecke was hospitalized after being beaten in Dresden, in one of the worst incidents.