Footage of the incident emerged on local news websites as European leaders express deep concern about the worsening situation on the streets of the Ukrainian capital, where thousands of people have massed in often violent clashes with authorities.
Protests against president Viktor Yanukovich began in November, when he pulled out of signing a free trade deal with the European Union in favour of closer economic ties with former Soviet overlord Russia.
Unrest has swelled in scale in recent weeks, and turned violent on Sunday when hard-core radicals broke away from the main protest area in the capital Kiev and clashed violently with riot police.
Three people have been killed on the side of protesters - two of them from gunshot wounds - and more than 150 police have been injured in the worst street violence in post-war Kiev.
A protest truce on Thursday night ended with anti-government protesters erecting more street barricades in Kiev after opposition leaders emerged empty-handed from talks with Mr Yanukovich.
Meanwhile, Ukrainian interior minister Vitaliy Zakharchenko has apologised and ordered officers to "stick to the norms of the law and morals" after a video surfaced of a member of the elite Berkut riot police force assaulting and humiliating a naked protester.
The footage shows man - identified by protest leaders as Mykhailo Havrylyak - being stripped completely naked except for his socks, being roughly pushed around and forced to stand still on the snow in the freezing temperatures while a police officer in a balaclava films him with a mobile phone.
He is also forced to pose for pictures clutching a spade, alongside another balaclava-clad officer who mockingly puts his arm around the prisoner.
This officer then hits him roughly on the back of the head before ushering him onto a bus.
Mr Zakharchenko swiftly apologised for the "unacceptable actions of the people in police uniform" saying an internal probe had been launched into the circumstances of the incident.
The head of the protest camp on Kiev's Independence Square, known locally has the Maidan, said Mr Havrylyak was being given medical treatment.
"Now he is [in] the Maidan self-defence headquarters and is being given treatment," Andriy Parubiy wrote on Facebook.
It is not clear who posted the video on YouTube or how long Mr Havrylyak spent in detention.
Meanwhile, opposition leader Vitaly Klitschko says a second round of talks with Mr Yanukovich on Thursday evening yielded nothing, and he voiced fears that the impasse could now lead to further bloodshed.
Scores of others on both sides have been injured - many of them with eye injuries caused by flying projectiles and police rubber bullets.
After speaking first to protesters manning barricades at the main confrontation point with police, Mr Klitschko then went to Kiev's Independence Square where he declared, "hours of conversation were spent about nothing".
"There is no sense sitting at a negotiating table with someone who has already decided to deceive you," he said.
"I earnestly wish that there will be no bloodshed and that people are not killed... I will survive, but I am afraid there will be deaths, I am afraid of this."
Three opposition politicians - Mr Klitschko, former economy minister Arseny Yatsenyuk and far-right nationalist Oleh Tyahnibok - had tried to wring concessions from Mr Yanukovich that would end the street protests.
In what could constitute the first signs of a willingness to compromise, Mr Yanukovich told parliamentary speaker Volodymyr Rybak that the "situation must be settled immediately."
Mr Rybak said the proposed emergency session of parliament could consider the opposition's call for prime minister Mykola Azarov's government to step down.
He said "questions linked to laws passed by parliament" could be discussed - apparently a reference to sweeping anti-protest laws rammed through parliament last week by lawmakers loyal to Mr Yanukovich.
Those laws served to boost mass demonstrations on the streets of Kiev at the weekend, and the opposition is demanding they be repealed.
In an initial round of talks on Wednesday, Mr Yanukovich refused to make any real concessions to opposition leaders' demands for the dismissal of his government and repeal of the anti-protest laws.
The mistreatment of the protester and failed talks came as German chancellor Angela Merkel voiced outraged at the new laws passed last week, which give the authorities in Ukraine new powers to suppress public protest.
"We are greatly worried, and not only worried, but also outraged at the way laws have been pushed through that call these freedoms into question," she told a news conference.
United States vice president Joe Biden spoke to Mr Yanukovich on Thursday and warned him that failing to de-escalate the standoff could have "consequences," the White House said.
"The vice president underscored that only the government of Ukraine can ensure a peaceful end to the crisis and further bloodshed would have consequences for Ukraine's relationship with the United States," the White House said.
French president Francois Hollande has urged Ukrainian leaders to rapidly seek dialogue with the opposition and former Soviet leader Mikhail Gorbachev is calling on the Russian and US presidents to get directly involved in negotiations to try and avert a "catastrophe".
Underlining the level of mistrust between the government and opposition, Mr Azarov accused protesters of an "attempt at a coup d'etat", and dismissed the possibility of an early presidential election to resolve the standoff.
On the sidelines of the World Economic Forum in Davos, Switzerland, Mr Azarov rejected comments from Russia that the situation in Ukraine is now out of control and says while the protesters' deaths should be investigated, evidence points to fellow demonstrators being responsible.
"First of all I would like to say that the situation in Kiev is absolutely controllable," he said.
"Secondly the country in general, the whole of Ukraine is working and operating in an absolutely normal way.
All those events we're talking about are happening just in the heart of Kiev.
"All those who support this coup should say clearly, 'yes, we are for the overthrow of the legitimate authorities in Ukraine', and not hide behind peaceful protesters."
Source: ABC News