A Ukrainian nationalist demonstration brought ugly scenes outside parliament in Kyiv as protesters clashed with police.
The protesters wanted, but failed to get, official recognition for a controversial World War II partisan group.
Proceedings in parliament were brought to a halt because of the trouble outside.
The far-right Svoboda party and the nationalist group the Right Sector both said their members were not responsible for the unrest.
Ultra nationalists and agent provocateurs have been blamed.
An interior ministry adviser was quoted as saying he could not rule out the idea that Russian intelligence was behind the trouble.
Amid increasing tension, smoke canisters and stones were thrown at police, windows were blown out by air guns.
The violence saw 50 arrests and left 15 policemen injured.
Euronews correspondent in Kyiv Dmytro Polonsky reported from the scene:
“The protesters outside the Verkhovna Rada (Ukraine’s parliament) are demanding that MPs pass a law recognising the soldiers of the Ukrainian Insurgent Army (UPA) as fighters for independence. Now militias are using tear gas and batons as they guard the building and the politicians.”
The Ukrainian Insurgent Army first fought but later allied themselves with the Nazis.
Many in Ukraine view them as collaborators – they were also accused of atrocities – but for the protesters they were heroes fighting for Ukrainian independence.
There have been several attempts to gain official recognition, all have failed.
“We should have recognised the Ukrainian Insurgent Army 20 years ago when there were 12,000 of us – now only a few hundred remain. But who needs it? We’ve been rehabilitated by history,” said a former UPA veteran dressed in military uniform.
The nationalists counted youth as well as the old among their supporters.
“If the people have recognised that the Organisation of Ukrainian Nationalists and the Ukrainian Insurgent Army are heroes who fought for Ukrainian independence, then the state should recognise it – otherwise there is a real gap between government and people,” a young woman added.
Tuesday evening saw nationalists again take to the streets of Kyiv, peacefully.
They also called for recognition for the UPA and better conditions for soldiers fighting in the government’s operation against pro-Russian rebels in eastern Ukraine.
The marchers, some of whom lit flares and set off fireworks, included far-right militia from the Azov Battalion, described by some as “patriots”, by others as “neo-Nazis”.