The BBC's David Stern said it is not clear what sparked the latest clashes
Violent clashes between protesters and police have erupted in Ukraine's capital, Kiev, with at least seven people, including a policeman, killed.
In the worst violence in weeks, police used rubber bullets and stun grenades to stop thousands of protesters marching on parliament.
A deadline set by the security forces for the violence to end has passed with no immediate sign of police action.
The clashes came as MPs were due to debate changes to the constitution.
The proposals would curb the powers of President Viktor Yanukovych, but the opposition say they were blocked from submitting their draft, meaning no debate could take place.
EU foreign policy chief Catherine Ashton said she was "deeply worried" by the escalation of violence, and urged politicians to "address the root causes".
Russia blamed the upsurge in violence on "connivance by Western politicians and European structures" and their refusal to consider the "aggressive actions" of radical factions within the protest movement.
Officer shot dead
Ukraine's unrest began in November, when Mr Yanukovych rejected a deal with the EU in favour of closer ties with Russia.
The mood had calmed in recent days, but protest camps remain on the streets and the opposition - which insists the president must resign - had warned the government it risked inflaming tensions if it failed to act.
On Tuesday, thousands of protesters tried to march on the parliament building to put pressure on the government to address constitutional reform.
But the march was blocked by lines of police vehicles.
The BBC's David Stern in Kiev says it is unclear what sparked the clashes - protesters and police have blamed each other.
Unlike in previous weeks, violence took place in a number of locations, our correspondent adds.
Some protesters ripped up cobblestones to throw at police. Others threw smoke bombs. Police fired stun and smoke grenades, and rubber bullets.
Protesters also attacked the headquarters of President Yanukovych's Party of the Regions, temporarily smashing their way in before being forced out by police.
One person - believed to be an employee - was found dead inside.
The bodies of three protesters were found inside a building close to parliament. Another three bodies were seen lying in the street.
The interior ministry said a policeman had died after being shot in the neck.
Demonstrators smashed their way into an office of President Yanukovych's Party of the Regions
The clashes are the worst seen in Kiev in weeks
A number of police and security force personnel were injured - and a policeman was killed - in the unrest
Dozens of protesters and security personnel are also reported to have been injured.
The heads of the security services and internal affairs ministry gave the protesters a deadline of 18:00 to put an end to the clashes, warning they would "use all the possible methods" to end it.
The entire Kiev metro has been shut down, and police have converged on the edges of Independence Square, the site of the main protest camp since November.
Protest leader Vitaly Klitschko urged women and children to leave the square, saying they could not "exclude the possibility of use of force".
But the deadline came and went with no apparent sign of security force action.
US 'appalled'
Inside parliament, there were scuffles as the opposition tried to submit a draft resolution on reinstating the 2004 constitution.
The changes would mean President Yanukovych losing some of the powers he has gained since his election in 2010, including the power to appoint the prime minister and most cabinet members. They could also lead to snap presidential elections.
Opposition leader Arseniy Yatsenyuk said the move was being blocked by President Yanukovych, saying his party members "show no desire whatsoever to end the political crisis".
MPs who support the president say the proposals have not been thoroughly discussed, and that more time is needed.
The speaker of parliament, Volodymyr Rybak, said opposition leaders would meet the president on Wednesday morning for further talks.
The US said it was "appalled" by the latest violence and called on the president to "immediately de-escalate the situation", while German Foreign Minister Frank-Walter Steinmeier called on both sides to "return urgently" to their attempts to find a political solution.
In a separate development, Russian Finance Minister Anton Siluanov said Moscow would provide Ukraine with a new $2bn (£1.2bn) tranche of loans this week.
In December, Moscow pledged $15bn to back Ukraine's struggling economy, but so far only $3bn has been transferred.
The Kremlin had hinted it would freeze the loan until a new government acceptable to Moscow was formed after Ukrainian PM Mykola Azarov resigned last month.
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