Ukrainian president Viktor Yanukovych has refused to meet Vitali Klitschko after the boxer turned opposition leader went to his office hoping for negotiations.
The snub dims hopes for a resolution to the political crisis that has escalated into vicious street clashes between protesters and police.
Ukraine's two-month standoff shifted into a new phase after Mr Yanukovych pushed through harsh anti-protest laws last week in an attempt to quash the demonstrations calling for his removal.
The mass protests in the capital Kiev erupted after Mr Yanukovych spurned a pact with the European Union in favour of close ties with Russia, which offered him a bailout worth about £9 billion.
Seeing the government ignore their demands and opposition leaders unable to present a coherent plan or even select a single leader, radical protesters have clashed with riot police in Kiev since Sunday, hurling stones and fire bombs at police and getting hit with tear gas and rubber bullets in return.
In a bid to ease the crisis, Klitschko, a heavyweight boxing champion, went to Mr Yanukovych's office for face-to-face talks, but was received only by his aides. The aides were part of a working group Mr Yanukovych told to hold negotiations with the opposition. Mr Yanukovych's office said the president was in a meeting at the time.
Klitschko held out hope that the leader would receive him later, saying: "The centre of the city of Kiev has been burning for two days. The president sits two blocks away and does not hear it."
Despite heavy snowfall and freezing temperatures, several thousand protesters remained camped outside a government district in Kiev in front of rows of helmeted riot police who hid behind metal shields as charred buses stood covered with ice.
The protesters managed to hold their ground overnight, even as police moved in to dismantle some barricades. Fighting eased this morning after Orthodox priests intervened and asked both sides for calm.
Still, nearly 1,500 activists sought medical help after the clashes, according to Oleh Musiy, who runs the protesters' medical team.
Opposition leader Arseniy Yatsenyuk, who had previously condemned violent protests, said the situation has changed.
"People have received the right to switch from peaceful to non-peaceful protest because the deafness of the authorities and their disregard for the people," he said.
His ally, the jailed former prime minister Yulia Tymoshenko, also urged Ukrainians to go to the barricades.
"There is no other way for the people to deal with the mafia. Those standing on the front lines for Ukraine are heroes," Ms Tymoshenko said in a statement.
During the clashes last night, Klitschko claimed that groups of young men had been hired by pro-government forces to smash shop windows and set cars ablaze in Kiev to create a pretext for the government to introduce martial law.
Protesters chased the men, captured them and took them to be questioned. In a video broadcast by several Ukrainian media organisations, the activists showed a hammer and other tools used by the assailants. The protesters then marched the captives down the street and forced them to apologise.