Dmytro Bulatov is a 35-year-old activist from the Avtomaidan group, car drivers who hold motorcade protests against Yanukovych. He was reported missing by his wife late yesterday, Kiev police said.
Bulatov's disappearance came after the two-month protests escalated into deadly clashes with police and as other activists were going into hiding or even leaving the country.
Activists are "really concerned about Dmytro Bulatov's disappearance," Sergiy Klein, another Avtomaidan member, told AFP, adding that he himself had left Ukraine for Germany yesterday.
Bulatov had told the Ukrainska Pravda website on Wednesday that his car and the front door to his apartment had been vandalised and he hinted that he expected further reprisals.
"If something happens to me, there would be even more outrage," he said.
Ukraine last week passed a slew of anti-protest laws, one of which makes it illegal for five cars to travel in a motorcade, apparently targeting Avtomaidan.
Avtomaidan has organised several motorcades of over a thousand cars that protested near Yanukovych's sprawling country estate in Mezhygirya outside Kiev.
Verbytsky was found dead in a forest near Kiev yesterday, while Lutsenko survived a severe beating.
Rights groups meanwhile accused Ukrainian police of maiming detained protesters and singling out journalists.
"Nothing can justify the vicious beatings we've documented," said Human Rights Watch researcher Yulia Gorbunova in a statement today. The group said riot police officers tortured a 17-year-old protester who had been taking pictures of this week's clashes on his cell phone.