Members of the nationalist group C14 have sprinkled pro-Russian Progressive Socialist Party leader Natalya Vitrenko with white liquid.
The incident took place outside the district administrative court in Kyiv.
"Communists headed by Vitrenko came to file a complaint with the district administrative court. They do not like being tormented by patriots 'with impunity' and being banned from waving red cloths on 9 May," the group said.
Its representatives left the site of the incident immediately.
Police are investigating the incident, their press service said.