STOCKHOLM, Dec 7 (Reuters) - Police were attacked with firebombs and rocks in a poor suburb in Sweden's capital late on Saturday, leading to the arrest of eleven young people for rioting and arson.

The events in Ragsved in southern Stockholm came after week-long riots in Husby on the other side of the capital in May last year when hundreds of cars were burnt as police battled immigrant youths after a Portuguese man was shot dead by police.

A police spokesman said it was too early to say who the perpetrators were this time, but that four of the eleven arrested were younger than 18.

"What is quite unusual here is that this was seemingly somewhat planned ahead," police spokesman Kjell Lindgren said.

He said firebombs and piles of paving stones were already prepared when police arrived and the approximate 30 people behind the riot were reported to have been masked.

No people were harmed but around 10 cars were set on fire. Police cars were damaged and a Ragsved police office also saw some damage.

Like Husby, Ragsved has a large immigrant population, and further violence in immigrant suburbs could help boost the far-right anti-immigration Sweden Democrats in snap elections in March next year.

The Sweden Democrats effectively brought the centre-left government down after just two months in office by breaking with common practice and voting for the centre-right opposition budget, thereby getting it passed.

Prime Minister Stefan Lofven said on Wednesday he would call for the country's first snap elections in over 50 years, to be held March 22 next year.

The Sweden Democrats made significant gains in the regular election in September, doubling their vote to become the third largest party. (Reporting by Daniel Dickson; Editing by Mark Potter)

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