STOCKHOLM (AFP) - Swedish police launched a manhunt on Thursday after the third arson attack against a mosque in a week, amid growing tensions over the rise of an anti-immigration far right movement.

"Someone threw a firebomb, a Molotov cocktail at the building," Mr Torsten Hemlin, a spokesman for Uppsala police, told Swedish news agency TT, adding that the mosque in eastern Sweden did not catch fire.

"They also wrote some vulgar racist words," he said, adding that no one was in the building at the time.

Sweden's Islamic Association posted a photograph online of the main door of the mosque which was emblazoned with an anti-Muslim slogan.

Police in Sweden's fourth largest city, Uppsala, were alerted by passers-by who reportedly saw a man throw a burning object at the mosque at around 0430 GMT (12.30pm Singapore time).

"The crime has been classed as attempted arson, vandalism and incitement to hatred," the police said in a statement appealing for eyewitnesses to come forward.

Thursday's attack came just three days after a late night blaze at a mosque in Esloev in southern Sweden which police suspect was also arson.

On Christmas Day, a week ago, five people were injured when a petrol bomb was thrown through the window of a mosque in Eskilstuna, east of the capital Stockholm.

Sweden’s leftist Prime Minister Stefan Loefven led condemnation of the latest attack.

“The most important thing now is that everyone distances themselves from this,” he told TT.

“In Sweden, no one should have to be afraid when they practice their religion,” he added, saying the government would increase funding for securing places of worship.

'PEOPLE ARE AFRAID'

According to the anti-racism magazine Expo, there have been at least a dozen confirmed attacks on mosques in Sweden in the last year and a far larger number are believed to have gone unreported.

"People are afraid, they fear for their safety," Mr Mohammad Kharraki a spokesman for Sweden's Islamic Association told AFP.

"We've seen through history that people use violence as a way of polarising the society against minorities."

The attacks come as debate intensifies in Sweden over immigration and the integration of asylum seekers in the traditionally tolerant Nordic country which is expected to receive more than 100,000 asylum applications this year, breaking all previous records.

Last month, the far right Sweden Democrats - which doubled its support to 13 per cent in September elections - came close to bringing down the left-green government over its liberal refugee policies. The party's support in opinion polls has risen to around 16 per cent.

However, in a last minute agreement on Dec 27, the government and centre right opposition parties cut a deal effectively denying the Sweden Democrats influence over major policy - including over immigration.

Mr Kharraki said the arson attacks could be carried out by "Sweden Democrats people who are angry because they've been pushed aside".

"They think Muslims are the problem," he said, while "mainstream political parties have taken a stand against racism and Islamophobia".

However, a spokesman for the Sweden Democrats said there was no reason to consider the attacks to be politically motivated.

"This is not political, it's criminal. It's criminals doing this and it's a police matter, not a political question," said Mr Henrik Vinge.

"This type of violence is something we take very seriously... It's unacceptable, of course."

Muslim groups have called on politicians to join vigils in several cities around the country on Friday to show their opposition to racially-motivated violence.