Dozens of masked men believed to belong to neo-Nazi gangs carried out a number of assaults on migrants in Stockholm overnight amid rising tension over immigration, Swedish police said today.

Police had beefed up their presence in the city centre, deploying anti-riot and helicopter units after learning that extremists were planning 'aggression on unaccompanied migrant minors' in the city late yesterday.

"I was passing by and saw a masked group dressed in black... start hitting foreigners. I saw three people molested,' the Aftonbladet daily quoted one witness as saying.

Police spokesman Towe Hagg said by midday police had not received any complaints of assault but one 46-year-old man was arrested after striking a plain clothed officer.

Three further people were briefly detained for public order offences and one more faces charges for carrying a knife.

As many as 100 people, their faces covered, had descended in the early evening on the Sergels Torg pedestrian square, a popular meeting point for young people, including unaccompanied migrants.

Aftonbladet quoted witnesses as saying the masked group targeted 'people of foreign appearance' and handed out leaflets urging the infliction of 'deserved punishment on children of the North African street'.

Internet site Nordfront, an online forum for the neo-Nazi SMR movement, said its 'sources' had revealed that around '100 hooligans' from the AIK and Djurgarden football clubs had gathered yesterday in order to 'sort out the criminals coming in from North Africa'.AFP