COPENHAGEN — After an anti-Muslim provocateur publicly desecrated the Quran in Copenhagen, demonstrations against him on Sunday and early Monday descended into violent clashes between protesters, who set about 70 fires in the streets, and the police, who made 23 arrests.

The unrest in the Norrebro district of the Danish capital began on Sunday after Rasmus Paludan, the founder of a tiny far-right party, tossed a book he claimed was the Quran into the air and let it fall to the ground.

Mr. Paludan has spent the past months touring Denmark and staging offensive protests against Muslims and immigrants, often in neighborhoods with large immigrant populations. Until recently, he had largely been ignored by mainstream Danish news outlets, but the riots have propelled him to the center of national attention.

Protesters shouted at Mr. Paludan on Sunday, pressing against the lines of police officers who were protecting him. Violence then broke out and spread into surrounding streets, much of it shown live on television.