Former Ukrainian Interior Minister and current opposition leader Yuriy Lutsenko has been hospitalized in intensive care after being injured in a clash with riot police.
Lutsenko's wife and his Third Ukrainian Republic organization said Lutsenko had suffered a concussion after being struck in the head early on January 11.
Photographs showed Lutsenko, 49, with his head bandaged and a patch over his right eye.
The Ukrainian parliament's human rights ombudsman said 11 protesters had been injured, with two of them hospitalized, including Lutsenko. The Interior Ministry said 20 security officers had been hurt.
The Kyiv prosecutor's office said it had launched an investigation into the actions of both protesters and police.
The clashes broke out after a court on January 10 sentenced three nationalists to six years in prison each for allegedly plotting to blow up a statue of Soviet Union founder Vladimir Lenin in 2011. The protesters say the court proceedings against the men were a sham.
WATCH: Protesters clash with riot police after verdict announced.
Lutsenko's wife, Iryna Lutsenko, described her husband's condition to the opposition Hromadske television channel.
"The doctors diagnosed a closed head injury, a concussion, three subcutaneous hematoma, and an open wound on the face. There was no breach of the skull, thank God," she said.
"He was conscious, but not the whole time. He asked for people not to take revenge, because the Berkut [riot police] just obey orders. So he is asking people not to take revenge."
Reports said the unrest overnight on January 10-11 occurred after protesters tried to block police vehicles from taking the three convicted men from the court building.
Protester Dmytro Bulatov said: "As we see, the authorities do not stop in their acts of repression, it is not enough for them to deprive us of our rights, they want to put people in jail, give them six-year prison terms, just because they were talking about their rights. And they want to break our bones."
Tensions remain high in Ukraine since President Viktor Yanukovych's abrupt decision late in November not to sign an association deal with the European Union, which would have set out the terms of closer ties between Ukraine and the Western bloc.
The government's rejection of the pact sparked mass protests in November and December, in what became the biggest antigovernment movement in Ukraine since the 2004 Orange Revolution.
Neighboring power Russia had opposed the EU agreement. Since Ukraine's rejection of the deal, Moscow has moved to strengthen its economic and political ties to Ukraine.
Pro-EU demonstrators remain encamped in central Kyiv, demanding the resignation of the government and new elections.
Lutsenko has a been leading organizer of the latest round of protests.
Lutsenko served as interior minister in the cabinet of jailed former Prime Minister Yulia Tymoshenko, a fierce rival of Yanukovych. He was a figure in the Orange Revolution that brought Tymoshenko to power, and is considered a close ally of the former prime minister.
Under Western pressure, Yanukovych in April 2013 pardoned Lutsenko, who had been imprisoned for four years on charges of embezzlement and ordering illegal surveillance.
His prosecution had been criticized by the European Union and the United States, who said it raised questions about selective prosecution and the rule of law in Ukraine.
Tymoshenko's prosecution has been criticized by Western states on similar grounds. She was convicted in October 2011 and sentenced to seven years in prison for abuse of office.
With reporting by AFP, Interfax, Reuters, dpa, and RFE/RL's Ukrainian Service