The main anti-government protest camp in Kiev was engulfed in flames as police moved in after a chaotic day of street battles in the capital that have left nine people dead.

Riot police advanced toward protesters in Independence Square late on Tuesday using water cannon and stun grenades after shutting down nearby subway stations and restricting traffic. Protesters responded by setting fire to tyres and barricades in the camp and throwing petrol bombs. Television pictures showed large areas of the camp on fire.

Maria Lebedeva, a journalist speaking to Al Jazeera after leaving the square, said that "everything that can be on fire is on fire". She added that there were many women and children still in the area, and many people were injured. "This is one of the most violent days in Ukraine's history," she said.

Thousands of protesters had filled Independence Square earlier in the day, sensing that Ukraine's political crisis was reaching a critical turning point. Seven civilians and two policemen were killed, and hundreds more injured, in earlier clashes on Tuesday, security sources and opposition leaders told Reuters news agency.

Barrages of tear gas were met by petrol bombs as police broke through anti-government barricades and forced protesters towards their main camp in Independence Square. The opposition leader, Vitali Klitschko, called on women and children to leave the camp over fears of a possible police assault.

The rioting on Tuesday began after opposition leaders accused pro-government factions in parliament of dragging their feet on a constitutional reform that would limit presidential powers - a key opposition demand.

Within hours of the clashes, the Russian foreign ministry said that the violence was a "direct result of connivance by Western politicians and European structures that have shut their eyes ... on the aggressive actions of radical forces".

In a statement, it urged the Ukrainian opposition to "refrain from threats and ultimatums and establish a substantive dialogue with the authorities with the aim of extricating the country from deep crisis".

Denouncing the "grave new escalation" in Kiev, EU foreign policy chief Catherine Ashton condemned "all use of violence, including against public or party buildings".

"I urge the leadership of Ukraine to address the root causes of the crisis," she added, calling for an urgent return to a parliamentary process.

Echoing Ashton's sentiments a number of Western countries, including the US and France, called for an end to the violence in Kiev and restraint by security forces.

The renewed clashes have piled more pressure on Yanukovich to strike a deal with the opposition and defuse a 12-week crisis.

Unrest broke out last year when Yanukovich spurned the free trade agreement with the European Union and opted for a $15bn package of Russian credits and cheaper gas to prop up Ukraine's ailing economy in November.

"Today is the new stage of the events in Ukraine. We are coming to a stage which we may call being at the brink of tragedy," Valentin Yakushik, a political science professor at the University of Kiev, told Al Jazeera.

"Now the official opposition cannot control the people protesting in the streets. They came to the tactics of vandalism, burning down some buildings, destroying cars, throwing stones, molotov cocktails. Very dangerous situation."