Prosecutors confirmed the two had died from bullet wounds. They are the first fatalities since protests began in November at the government’s rejection of a planned treaty with the EU.
Wednesday’s clashes began after police moved in to dismantle a protest camp.
Ukrainian President Viktor Yanukovych is reportedly meeting with opposition leaders later to discuss the crisis.
The clashes come on the day that new anti-protest laws come into force. Parliament approved the laws last week, triggering renewed protests, which spilled into violence on Sunday night.
Hundreds of people have been injured, though some of the violence has been blamed on a little-known far-right group, Right Sector.
Meanwhile, the US embassy in Ukraine says it has revoked the visas of “several Ukrainians who were linked to the violence”. It did not give names, but said it was “considering further action against those responsible for the current violence.”
Wednesday’s violence began in a small area around Hrushevskyy Street, a road leading to government buildings and also close to the main protest encampment at Maidan (Independence Square).
Shortly after 08:00 a.m. – after a relatively peaceful night – police stormed the protesters’ barricades on Hrushevskyy Street.
The police later fell back to their positions after fierce clashes with protesters, but by the afternoon had pushed on through the barricades.
Protesters again hurled petrol bombs and stones while riot police responded with stun grenades and rubber bullets.
Meanwhile thousands of protesters have gathered in Independence Square.
There was a crush at one of the narrow entrances into the square when protesters trying to get in met protesters who were trying to get out to fight the police, our correspondent says.
At least two ambulances were seen carrying away the wounded.
Black smoke caused by the burning of tyres is now billowing over Kiev, while footage shows armoured vehicles moving into the area.
Officials confirmed the deaths of two people, found with gunshot wounds earlier on Wednesday. The general prosecutor said their bodies were found in a national library close to the scene of the clashes.
Medics for the activists say at least one person had multiple wounds and claimed he had been killed by a police sniper.
A third activist is also reported to have died on Wednesday from injuries sustained after falling from the top of the Dynamo football stadium.
PM Mykola Azarov denied the police would have been responsible for the deaths as they were not carrying live ammunition.
They “remain on the consciousness and responsibility of the organizers and certain participants of mass disturbances,” he said.
Many of the protesters have been wearing helmets and face masks in defiance of the new laws that ban the wearing of such headgear at protests. The laws also prescribe jail terms for anyone blockading public buildings and outlaw unauthorized tents in public areas.
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