Ukraine's interim President Olexander Turchynov has condemned the ultra-nationalist Right Sector, saying the group is bent on "destabilisation".
Right Sector activists blocked the parliament (Rada) building in Kiev on Thursday night and smashed windows.
They blamed the interior minister for the killing of a Right Sector leader.
Meanwhile, ousted President Viktor Yanukovych has called for a national referendum to determine each region's "status within Ukraine".
He fled to Russia last month after massive demonstrations against him and clashes between protesters and police in which more than 100 people died. The Kremlin says the new government in Kiev came to power illegally.
"As a president who is with you with all my thoughts and soul, I urge every sensible citizen of Ukraine: Don't give in to impostors! Demand a referendum on the status of each region within Ukraine," said Mr Yanukovych, quoted by Itar-Tass news agency.
In his first comments since Crimea voted to become part of Russia, Mr Yanukovych denounced fresh presidential elections planned for 25 May.
Meanwhile, Russia's defence minister, Sergei Shoigu, said that all Ukrainian service personnel loyal to Kiev have now left Crimea and all military installations there are under Russian control.
'Attempt to destabilise'
At a parliament session on Friday, Mr Turchynov, called the Right Sector rally outside parliament "an attempt to destabilise the situation in Ukraine, in the very heart of Ukraine - Kiev. That is precisely the task that the Russian Federation's political leadership is giving to its special services".
Right Sector activists are furious over the death of Oleksandr Muzychko, better known as Sashko Bily, one of their leaders. The interior ministry said he died on Monday night in a shoot-out with police in a cafe in Rivne in western Ukraine.
A member of the far-right group in Rivne threatened revenge for the killing of Mr Muzychko.
"We will avenge ourselves on [Interior Minister] Arsen Avakov for the death of our brother. The shooting of Sashko Bily is a contract killing ordered by the minister," Right Sector member Roman Koval was quoted as saying by the Ukrayinska Pravda website.
The Right Sector played a prominent role in the Kiev protests - and the clashes with police - which led to the removal of Mr Yanukovych from power. Its main support base is in western Ukraine.