Opposition protesters were Monday locked in a tense standoff with Ukrainian security forces in Kiev after hours of unprecedented clashes deep into the night left dozens wounded and parts of the centre resembling a battlefield.

The clashes, the worst in Kiev in recent times, came amid mounting anger over new restrictions on protests ordered by President Viktor Yanukovych after almost two months of protests over his refusal to sign a pact for integration with the EU.

A special commission set up by Yanukovych was due Monday to meet representatives of the opposition for emergency talks but it was unclear if this could in any way help ease the crisis.

In near apocalyptic scenes close to parliament, several police buses and vehicles were torched by the protesters who hurled stones and Molotov cocktails at the ranks of the security forces. Police responded with tear gas, stun grenades, rubber bullets and water cannon.

The White House urged an end to the violence, with US National Security Council spokeswoman Caitlin Hayden saying that Washington was deeply concerned and urging "all sides to immediately de-escalate the situation".

The spokeswoman warned that Washington was still considering sanctions against Ukrainian officials, a step urged by the Ukrainian opposition. "The US will continue to consider additional steps -- including sanctions -- in response to the use of violence."

After intense clashes continued into the early hours of Monday morning, the situation was calmer at 0700 GMT but hundreds of protesters who had spent the night in temperatures of minus 10 degrees Celsius were still out on the streets.

However the situation remained tense with protesters launching occasional sorties at the police line to throw stones or Molotov cocktails.

After a peaceful mass rally in the afternoon, hundreds of demonstrators sought to storm police cordons near the Verkhovna Rada parliament and close to the stadium of the legendary Dynamo Kiev football club in central Kiev, witnesses and AFP correspondents said.

In the most violent scenes since the start of the protests in November, demonstrators set five buses and two trucks on fire while the air filled with the stench of tear gas.

Their faces covered by scarves or ski masks, many of the protesters wielded sticks or even chains. Later in the night, they began to dig the cobble stones out of the road to hurl at police and use as barricades.

The security forces made extensive use of water cannon in a bid to douse the protesters and push them back and used rubber bullets which activists said left dozens injured.

Ukrainian opposition television broadcast pictures of two young men who it said were stripped naked by the security forces and then peppered with rubber bullets.

Health officials said 24 people were injured and three were hospitalised, while police said more than 70 officers had been hurt. The interior ministry said 20 people had been arrested for mass rioting.

Opposition leaders, including former boxing champion Vitali Klitschko and Arseniy Yatsenyuk, called on the protesters to refrain from using force but their calls were ignored.

It was not clear who was behind the clashes with police, which appeared to have been a well-organised move. Ukrainian media linked the action to a hitherto little-known right-wing youth group called "Right Sector".

Amid the chaos of the clashes, Klitschko was sprayed with powder from a fire extinguisher, leaving his eyes irritated and face and clothes covered in white powder.

In an apparent attempt to find a compromise, Klitschko travelled to the president's luxurious Mezhygirya residence outside Kiev to meet Yanukovych in person.

The president received Klitschko and promised early Monday to create a special commission of officials set up by national security council secretary Andriy Klyuyev to solve the crisis, the boxer's party and the presidency announced. The presidency said the new commission would meet the opposition on Monday.

Klitschko told online television channel Hromadske TV that the president had appeared "very concerned" by the latest events but also pointedly ignored the opposition's main demand for early elections.

In the afternoon, some 200,000 people had filled Kiev's Independence Square and surrounding streets for a new mass rally in defiance of new strict curbs on protests passed by lawmakers in a show of hands last week and signed into law by Yanukovych.

The new laws allow the authorities to jail those who blockade public buildings for up to five years and permit the arrest of protesters who wear masks or helmets. Other provisions ban the dissemination of "slander" on the Internet.

Protesters expressed frustration at the rally over the lack of a clear programme from the opposition leaders after almost two months of protests, whistling and heckling the opposition leaders during the main rally for their perceived inability to mount a stronger challenge.

Yanukovych's arch nemesis Yulia Tymoshenko remains in jail, while the protest leadership appears riven by rivalries ahead of presidential election next year.