Right Sector activists replaced EU flags at the Lviv administration with their own red and black banners, local news outlet Podrobnosti.ua reported.
Right Sector followers announced an indefinite rally in Lviv in support of the group's members who had clashed with police in the Western Ukrainian town of Mukachevo. They also blocked an international highway to a stop special task force from entering the town.
The group's members set two tents with their symbols near the administration building, but they were removed later.
Mukachevo, located in the Zakarpattia Region, turned into a flash point on Saturday after a skirmish between Right Sector militants and local police erupted. According to the Ukrainian Interior Ministry, the fighters started the shootout and broke through entrapment.
From the fighters' point of view, the police opened fire without warning and the Right Sector members "had to defend themselves."
Two people were reportedly killed and 11 wounded. A criminal case was initiated.
Right Sector supporters staged a wave of rallies in a number of Ukrainian towns demanding the resignation of Interior Minister Arsen Avakov and the police chiefs of the Zakarpattia Region.
The Right Sector is a hardline paramilitary nationalist union that rose on the wave of the Maidan turmoil last year. The group is notorious for crimes such as the Odessa massacre and has repeatedly defied obeying authorities' orders.