Kiev: A mob of Ukrainian nationalists attacked police with rocks, firecrackers and chains in an angry protest outside parliament on Tuesday in what President Petro Poroshenko called a "planned" provocation.
Interior Minister Arsen Avakov said 15 policemen were injured in the violence and 37 people were detained.
Far-right supporters have traditionally held a march through Kiev on October 14 to mark the founding of the Ukrainian Insurgent Army (UPA), a paramilitary partisan movement formed in 1943 to battle for independence against Polish, Soviet and German forces in western Ukraine.
The UPA is historically controversial, idolised by Ukrainian nationalists but despised by Russia for collaborating with Nazi forces and fighting the Soviet army.
Poroshenko on Tuesday declared October 14 a national holiday to commemorate "defenders of Ukraine" and cancelled a Soviet-era holiday for "defenders of the fatherland" on February 23 in a move to shed Ukraine`s Soviet legacy.
Parliament was also due to debate a bill calling for the UPA to be recognised as independence fighters during World War II but lawmakers failed to include it on the agenda.
Hundreds of men, some masked, then attacked the line of police on guard outside the parliament building in Kiev.
Poroshenko said the clashes were organised by an outside force, hinting at Russia, which Kiev has blamed for stirring a bloody insurgency in the east.
"I will not let provocateurs, deserters and paid agents of another state open a second front inside the country," he said.
The hardline nationalist Right Sector group, which is known for staging violent protests, said its activists had nothing to do with Tuesday`s trouble.