A man armed with a kitchen knife has attacked passengers on a crowded bus in the northern German city of Luebeck before being overpowered and arrested.

Ten people were injured, three of them seriously.

Authorities had no immediate information on the assailant’s motive but they believe the suspect had no background in terrorism.

Investigators found a flammable substance in a backpack on the bus but no explosives.

The incident started when the assailant set fire to the backpack, prosecutor Ulla Hingst said at a Friday evening news conference.

The driver told investigators that he stopped the bus and opened all the doors to let passengers out after noticing the fire in his rear-view mirror.

He then walked back to find out what was going on and was hit by the assailant, Ms Hingst said.

As the suspect left the bus, he stabbed people around him with a 5in kitchen knife. The attacker was overpowered by passengers outside the vehicle, then quickly arrested by police nearby.

Local officials at the sceneHans-Joachim Grote, left, an interior minister for Schleswig-Holstein, and Jan Lindenau, mayor of Luebeck, at the scene (AP)

Five of the victims were still hospitalised on Friday evening.

The suspect is a 34-year-old man who lives locally. Ms Hingst said he is a long-time German citizen who was born in Iran.

“There are no indications that the man was in any way politically radicalised,” Ms Hingst said. “For that reason we have, as the investigation currently stands, no indications that there was a terrorist background to this act.”

She added the motive is “still open” and “we are investigating in all directions”.

Prosecutors will seek to have the man kept in custody pending possible charges of bodily harm and attempted arson, Ms Hingst said.

The bus was travelling from Luebeck to the neighbouring seaside resort of Travemuende, where an annual regatta, the Travemuender Woche, was opening later on Friday.

Schleswig-Holstein’s state interior minister Hans-Joachim Grote said there was no reason to believe that there was an increased risk to the regatta but police presence there was stepped up as a precaution.