A group of youths who have been torching cars and waste containers in the Danish capital said the wave of unrest started as a protest against police harassment.
In a letter published in Copenhagen newspaper Politiken, the youths accused police of "brutal, racist" behavior.
It was the first time that rioting juveniles had offered any explanation for the fires that have raged for nine consecutive nights mostly in immigrant neighborhoods across the country. The authors of the letter said they would now stop rioting.
"Basically the unrest is about the way we are treated by the police, who are brutal, racist and totally unacceptably insulting," the group said in the letter.
The authors called themselves "Boys of inner Nørrebro," referring to the immigrant neighborhood in Copenhagen where the unrest started on Feb. 10.
Police said 21 fires were set overnight Tuesday for a total of about 600 since the wave of arson started. The youths have burned cars, trash containers and schools. In some cases, they have pelted firefighters with rocks, but no serious injuries have been reported.
Prime Minister Anders Fogh Rasmussen defended police and firefighters and put the blame on the rioting youths and their parents.
"The youths have a responsibility and now they must stop. Their parents also have a responsibility," he told reporters Tuesday.
Fogh Rasmussen said he had asked the Danish justice minister to examine whether the parents could be held financially responsible for damage caused by the fires.
Some observers have said that the reprinting of a caricature of the Prophet Muhammad may have aggravated the situation.
However, Tuesday's letter did not mention the prophet drawing, which Danish newspapers reproduced Wednesday in a gesture for free speech following the arrest of three men accused of plotting to murder the cartoonist.
Copenhagen police chief Hanne Bech Hansen said she met Monday with the youths who wrote the letter, and would investigate their concerns about police harassment.