Stockholm (AFP) - Dozens of masked men believed to belong to neo-Nazi gangs gathered in Stockholm and handed out leaflets calling for attacks against young migrants, Swedish police said Saturday, amid rising tensions over the country's refugee influx.

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 on Friday.

"I was passing by and saw a masked group dressed in black... start hitting foreigners." 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 plainclothes officer.

Swedish Interior Minister Anders Ygeman condemned the "racist groups which threaten and spread hate in the public space", adding that "one must respond strongly".

"This is a disturbing trend in society," Ygeman said in a commentary published by news agency TT.

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

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

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."

The website 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 Friday in order to "sort out the criminals coming in from North Africa."

After initially taking a generous stance on migration -- the country of 9.8 million is among the European Union states with the highest proportion of refugees per capita -- Sweden has in recent days said it expects to expel tens of thousands of people over several years as it struggles to cope with the influx.

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Booed by dozens of "anti-fascist" activists, some 200 people gathered in Stockholm on Saturday to demand the resignation of Prime Minister Stefan Lofven as well as the acceleration of migrant deportations, an AFP journalist said.

The number of new migrants entering the country has plunged since Stockholm on January 4 introduced systematic photo ID checks on train, bus and ferry passengers coming in via Denmark.

The toughening of policy comes against a backdrop of rising concern over conditions in the country's overcrowded asylum facilities.

Officials also called for greater security after an employee at a refugee centre for unaccompanied youths was fatally stabbed earlier this week.