Ukraine ProtestsREUTERS/Gleb Garanich Pro-European protesters stand in line holding wooden sticks during clashes with riot policemen in Kiev January 22, 2014. The European Union threatened on Wednesday to take action against Ukraine over its handling of anti-government protests after three people died during violent clashes in Kiev.

Ukraine's opposition agreed Thursday to observe an eight-hour truce in clashes with security forces after five days of deadly fighting but threatened to go on the attack if the government failed to agree concessions.

Opposition leader and world boxing champion Vitali Klitschko brokered the truce after talks with radical protesters and armored security forces on the frontline of the clashes, saying the ceasefire should hold while he conducts talks with President Viktor Yanukovych.

The fighting, which activists say has left five protesters dead, raged into the night at the epicenter of the clashes on Grushevsky Street in central Kiev, with demonstrators hurling Molotov cocktails and security forces using stun grenades.

The bloody clashes, which have turned parts of the usually placid capital into a war zone, marked a new peak in tensions after two months of protests over the government's failure to sign a deal for closer integration with the European Union under Russian pressure.

"By 8:00 pm (1800 GMT) I will return to you and inform you of the result of the talks," the Interfax Ukraine news agency quoted Klitschko as telling the protesters.

"Keep the barricades in place but (be) calm until the talks finish," he added.

An AFP correspondent at the scene of the fighting on Grushevsky Street confirmed that there had been a pause in clashes and that the truce appeared to be holding.

Klitschko and other opposition leaders are due to meet Yanukovych at the presidential administration this afternoon for a second round of talks.

The opposition has said the president must agree to three key demands -- the holding of snap presidential elections, the resignation of the government and the annulment of anti-protest laws passed last week -- for a compromise to be reached.

The protesters have marked their frontline with a semicircle of burning tyres which have sent rancid plume of black smoke billowing into the Kiev sky and are visible throughout the city.

However under the terms of the truce, protesters allowed police to douse the fires with water cannon and now only white smoke was rising at the scene.

'Ready for a bullet to the head'

Tens of thousands Wednesday evening had filled Independence Square in Kiev which was the main protest hub for the last two months, hoping their sheer numbers would deter any attempt by police to disperse the rally.

They sought to reinforce the protest barricades by several metres by filling sandbags with snow, turning the protest zone around Independence Square into a virtual fortress.

The leader of the opposition Fatherland party Arseniy Yatsenyuk had warned the protesters that Yanukovych had 24 hours to agree a peaceful solution, saying he was ready to die for the cause.

"If he does not go down that path then we will go forwards together and if it means a bullet to the head, then it is a bullet to the head.

Klitschko told crowds on Independence Square that protesters will go "on the attack" if Yanukovych does not swiftly offer concessions.

Oleg Musiy, the coordinator of the protest medical service, told pro-opposition Hromadske radio, that five people have been killed and around 300 wounded in Wednesday's clashes.

According to the Ukrainska Pravda news website, four of the five people found dead had gunshot wounds.

Meanwhile a prominent Ukrainian activist and journalist, Igor Lutsenko, on Wednesday appeared in public after being abducted from a hospital by unknown individuals and dumped in a forest outside Kiev.

However a man abducted with him, an activist named Yuriy Verbytsky has not been found, and a relative told Ukrainian media that he had been killed in a forest and she had identified his body in a local morgue.

'This is the end'

A former MP from Yanukovych's Region's Party who switched sides during the protest, Inna Bogoslovska, bluntly told the rally on Independence Square the authorities were doomed: "Viktor Fyodorovich, this is the end," she said.

But Prime Minister Mykola Azarov, who is at the World Economic Forum in Davos where he faces a chilly reception, warned that "any anti-constitutional actions in the capital have to be stopped".

He reaffirmed however that the government was ready to meet the opposition halfway.

"There are some demands (from the opposition) that could be a basis for a compromise but from our side there are demands that have to be fulfilled."

The deadly violence horrified Ukrainians, who have never witnessed such scenes in their country including during the 2004 Orange Revolution which was almost entirely peaceful.

Amid calls for sanctions against the Ukrainian government, European Commission has warned of "possible actions" against the Ukrainian authorities while the United States also revoked the visas of several Ukrainian nationals linked to violence against protesters in November and December last year.

Russia, which has regarded Ukraine's pro-EU protest movement with suspicion from the start, has taken a different view and blamed the opposition and West for the clashes.

But President Vladimir Putin's spokesman Dmitry Peskov said Thursday that Russia will not intervene in the protests and believes Ukraine's leadership will find a way out.