Ukraine's president said Friday a tentative deal had been clinched to end a three-month-old standoff with opposition protesters after a day of raging violence left dozens of people dead and turned the heart of Kiev into a war zone.

After marathon talks with President Viktor Yanukovych and opposition leaders, the European Union confirmed that a "temporary" agreement was due to be signed Friday in what would be a major step forward in ending the worst crisis since Ukraine's independence.

Yanukovych announced he was calling an early presidential election, without giving a date, as well as starting a process to reform the constitution and form a unity government.

The moves go some way towards meeting key opposition demands but it was not known yet if they would back the deal and EU ministers involved in the talks urged caution.

In the deadliest day since the crisis erupted in November, at least 60 people were killed in fierce clashes in Kiev's Independence Square on Thursday, with police opening fire on protesters carrying makeshift shields while opposition medics said government snipers picked off demonstrators from rooftops.

Three EU foreign ministers and a Russian envoy flew in for emergency talks on Thursday as the international community voiced increasing alarm about the crisis in the former Soviet state.

"The parties agreed on the initialling of an agreement to resolve the crisis," the presidency said in a statement following a day and night of negotiations between Yanukovych and envoys from Poland, Germany, Russia and France.

It had said the deal was to be signed at 1000 GMT but the time passed without any formal announcement.

Poland's Foreign Minister Radoslaw Sikorski said on Twitter that the talks were at a "delicate moment" adding: "All sides need to remember that compromise means getting less than 100 percent."

Under the expected terms of the pact, the country would revert back to the 2004 constitution, which would give more power to parliament and government, and less to the president.

There has been no comment yet from any of the top three opposition leaders.

But a senior EU diplomat involved in the negotiations said a "temporary agreement" was expected Friday.

The crisis flared in November when Yanukovych rejected an EU integration deal in favour of closer ties with historical master Russia, angering pro-EU parts of the population who rose up in protest.

The demonstrations had been largely peaceful, except for a brief outbreak of violence at the end of January, but the situation descended into deadly mayhem on Tuesday.

Protesters had marched on parliament in a 20,000-strong peaceful rally aimed at pressuring the government into agreeing to their demands, but it descended into violence that lasted throughout the day and night.

- 'Crime against humanity' -

Kiev authorities have put the death toll from the past few days at 77, but opposition medics say more than 60 protesters were shot dead by police on Thursday alone, which would make the toll considerably higher.

On Friday, some 20,000 protesters congregated on Independence Square, which has been occupied by activists since November and still remains under opposition control despite a brief yet deadly raid by riot police earlier in the week.

While the atmosphere on the square was calm -- and life elsewhere in Kiev creeped back to normal with the reopening of the city's metro system -- protesters told AFP the deal was too little, too late.

"These steps were what we needed but I think it is now too late after all the blood that has been spilled," said Sergei Yanchukov, 58.

"It was a crime against humanity and Yanukovych should be sent to The Hague (home of the International Criminal Court).

"At the very least he should resign immediately and be brought to justice."

The latest violence erupted Thursday -- on what was meant to be a day of mourning for those killed earlier in the week -- when protesters launched an offensive against riot police to kick them off the square.

Armed with Molotov cocktails, batons and paving stones, they pushed back the feared Berkut riot police, who responded with rubber bullets, stun grenades and Kalashnikov rifles.

Footage of the bloodbath that followed shocked the international community.

The White House said it was "outraged by the images of Ukrainian security forces firing automatic weapons on their own people".

Volunteer medics, who made a makeshift morgue at a popular hotel overlooking the square, also accused police of killing demonstrators with live rounds.

Ukraine's interior ministry responded that it reserved the right to use live munitions "in self-defence".

The ministry also accused "extremists" of seizing 67 of its troops at gunpoint and holding them hostage.

- EU sanctions -

The shocking scale of bloodshed in the nation of 46 million sandwiched between Russia and the European Union prompted EU officials to slap travel bans against those responsible for ordering the use of force.

Italian Foreign Minister Emma Bonino said an agreement was also reached at an emergency EU meeting in Brussels to impose asset freezes on those with "blood on their hands".

US Vice President Joe Biden also warned Yanukovych that Washington was ready to impose sanctions on officials guilty of ordering troops to fire on protesters.