Petro Poroshenko, a pro-Western businessman and member of Parliament, wrote on Facebook, “The attempt on the life of Tetyana Chornovol leaves no words, only rage and shame.” He added, “The Ukrainian people will not be intimidated.”

Mr. Zakharchenko, the interior minister and a close ally of Mr. Yanukovich, has faced a barrage of criticism over a violent crackdown by the police on peaceful demonstrators in Independence Square on Nov. 30.

Although lower-level officials have been disciplined in connection with the violence, their role is not clear. Mr. Zakharchenko is the most senior government official with direct authority over the police units involved in the enforcement action, and there have been repeated calls by the opposition for his dismissal.

In response to Ms. Chornovol’s beating, several protest leaders called for picketing outside the Interior Ministry headquarters. Mr. Yanukovich’s office issued a statement on Wednesday afternoon saying he had ordered the Interior Ministry and the general prosecutor’s office to investigate the attack.

The authorities on Wednesday said they had detained two men, but did not identify them.

Footage from a video camera on the dashboard of Ms. Chornovol’s car that was posted on the news website Ukrainska Pravda showed a prolonged cat-and-mouse chase, with a Porsche Cayenne sport utility vehicle trying to block the path of her much smaller vehicle, then repeatedly chasing her down and ramming into her. Eventually, she was forced to the side of the road.

Although Ms. Chornovol has been involved in a number of episodes involving high-profile activists, she is most famous for an incident in August 2012 in which she scaled the walls of Mr. Yanukovich’s residential compound, which includes 345 acres of forested hills along the Dnieper River and is called Mezhyhirya, after the park where it is situated.