A Palestinian man who allegedly stabbed one person to death and wounded six others in Hamburg was known to authorities as a suspected Islamic radical but also was psychologically unstable, German officials said.

Key points: Attacker is not named by authorities in keeping with Germany privacy laws

Attacker is not named by authorities in keeping with Germany privacy laws Suspect grabbed kitchen knife from supermarket shelf and stabbed three men, one of them fatally

Suspect grabbed kitchen knife from supermarket shelf and stabbed three men, one of them fatally Officials said he was on their radar as suspected Islamic radical, but not as a "jihadist"

The suspect, a 26-year-old who had no identity papers other than a birth certificate showing he was born in the United Arab Emirates, was quickly overwhelmed by passers-by and arrested after Friday's attack at a supermarket in Hamburg's Barmbek district.

He was not named by authorities in keeping with Germany privacy laws.

The man's motive remained unclear, but he is believed to have acted alone and there are no indications he had links to any network, Hamburg's state Interior Minister Andy Grote said.

Police said the suspect grabbed a kitchen knife with a 20-centimetre blade from a supermarket shelf on Friday afternoon and stabbed three men, with a 50-year-old man dying of his injuries.

He then left the supermarket and hurt another three people outside, not all of them with the knife.

Passers-by then pursued and overwhelmed the suspect, who was arrested by police.

Mr Grote said none of those injured was being treated for injuries considered life-threatening on Saturday, though some were seriously hurt.

German Chancellor Angela Merkel, who is on a summer vacation and is seeking a fourth term in office in September, praised the courage of the Hamburg residents who threw chairs and other objects at the attacker, helping police detain him.

"This violent crime must and will be investigated," she said in a statement.

"I thank the police for their effort and all those who stood up against the attacker with civil courage and bravery."

Suspect had asylum bid denied

The man arrived in Germany in March 2015 after stops in Spain, Sweden and Norway.

His asylum request was rejected late last year and authorities were trying to secure new Palestinian papers to deport him — a process in which they said he had cooperated.

Officials said he was on their radar as a suspected Islamic radical, but not as a "jihadist".

A friend had tipped authorities off to changes in the man, telling them he stopped drinking alcohol and started talking about the Koran, Torsten Voss, head of the Hamburg branch of the domestic intelligence agency, said.

Officials interviewed the man and came away with the impression he was a "destabilised personality" but not someone who posed an immediate danger, Mr Voss said.

"We evaluated him rather as someone who was psychologically unstable than had clear Islamic extremist motivations," Mr Voss said.

Authorities do not know of any connections to Hamburg's Islamic extremist scene.

Prosecutors said the attacker appeared on police records in April after he was caught stealing in a shop, but he was convicted of no crime given the petty nature of his offence.

A search of the man's room at a centre for asylum seekers turned up no weapons or weapon-like objects, prosecutors said.

The suspect has not yet talked about Friday's attack, prosecutor Joerg Froehlich said, though he has indicated he acted alone.

Mr Froehlich said authorities intend to ask that he be held in custody on suspicion of murder and five counts of attempted murder, but may seek to have him held at a psychiatric unit instead.

AP