A wave of vandalism continued across Denmark for a seventh consecutive night, officials said Sunday.

Thirteen people were arrested on suspicion of vandalizing and torching schools, cars and garbage containers, with 93 small fires burning overnight, police said. First preliminary reports said 11 people had been arrested.

The unrest — largely in immigrant neighborhoods — has spread through Copenhagen and across Denmark.

Police are unsure what triggered the violence but some say youths from Denmark's immigrant communities were protesting against perceived police harassment, and suggested Wednesday's reprinting of a cartoon lampooning the Prophet Muhammad may have aggravated the situation.

"Many people also think it's because the youths are bored," police spokesman Jan Marker said. "We're in the process of putting a profile of these youngsters together to see if we can come up with what is driving them. ... But the only thing we know for sure right now is that most of them are under 18."

Most of the unrest was reported in Copenhagen, particularly in the southwest, Marker said.

Ten buildings were set on fire, of which at least five were schools.

Parts of a school in Tingbjerg in Copenhagen was demolished after flames tore through staff rooms and other administrative offices.

"But the general picture is that it looks calmer," Marker said. "It's not as bad as the previous night. It could be because of the arrests we've made."

On Wednesday, more than a dozen Danish newspapers reprinted a cartoon that had sparked protests in Muslim countries two years ago in a gesture of solidarity after police revealed an alleged plot to kill the cartoonist.

In a statement Sunday, the Mecca-based Muslim World League urged Muslims, Islamic organizations and centers in Denmark to maintain calm following the reprinting.

"Muslims should not get caught in a whirlwind of emotional reactions," it said.

The organization, founded by the Saudi Arabian government and comprising Sunni religious figures from at least 22 countries, also called on responsible parties in Denmark to take the necessary measures to stop what they described as the offense against the prophet.