STOCKHOLM (REUTERS) - Police in the Swedish capital are to seek reinforcements after youths again set cars ablaze and threw stones at police for a fifth night running, officials said on Friday.

Around 30 cars were set on fire in poorer neighbourhoods in northwestern and southwestern parts of the capital on Thursday night and rioters caused widespread damage to property, including schools, police said.

Despite Sweden's reputation for equality, the rioting has exposed a fault-line between a well-off majority and a minority, often young people with immigrant backgrounds, who cannot find work, lack education and feel marginalised.

"In terms of extent, it is a little less, a little quieter," police spokesman Kjell Lindgren said of the disturbances on Thursday night. Eight people, mostly in their early 20s, had been detained during the night.

He said police were planning to request reinforcements from other areas to help deal with the rioting, upcoming football matches and the wedding of Princess Madeleine, third in line to the throne, on June 8.

He said the police needed to be prepared to maintain a heavy presence on the streets. "We will do that for days, weeks, as long as it is necessary," he said.

The unrest has led the United States embassy to warn US citizens this week not to go to areas hit by rioting.

"I can confirm we have sent out a Warden message," embassy spokeswoman Danielle Harms said, referring to alerts by the Department of State with safety or travel information.

The violence of recent days appears to have been sparked by the death in Husby - the centre of the rioting - of a 69-year old, shot by police earlier this month.

One recent government study showed up to a third of young people aged 16 to 29 in some of the most deprived areas of Sweden's big cities neither study nor have a job.

The gap between rich and poor in Sweden is growing faster than in any other major nation, according to the OECD, though absolute poverty remains uncommon.