Ukrainian President Viktor Yanukovych was on Monday holding a new and previously unannounced meeting with the three main opposition leaders aimed at resolving the country's crisis, the presidency said.

Along with senior aides, Yanukovych was meeting UDAR (Punch) party leader and world boxing champion Vitali Klitschko, Fatherland leader Arseniy Yatsenyuk, and nationalist Svoboda (Freedom) party leader Oleg Tyagnybok.

The meeting came after the country's main opposition parties said they were ready to continue talks with Yanukovych's government but warned that the patience of protesters in the streets could "snap" at any moment.

Opposition leaders said in a joint statement that they were ready to negotiate "despite an attempt by the authorities to abandon the negotiations and declare a state of emergency".

However, Jailed Ukrainian former prime minister Yulia Tymoshenko urged the opposition not to accept the "humiliating" terms of Yanukovych's deal.

"I ask the opposition leaders to categorically not accept the humiliating conditions," said the 2004 Orange Revolution co-leader, whose plight has only been raised on occasion during the current protests. "I ask all Ukrainians to go forward, not to stop and not to weaken the force of their blow on the authorities," she said in a statement on her website.

Earlier on Monday, Ukraine said the introduction of a state of emergency was not yet on the agenda after radicals seized the justice ministry in Kiev in an attack that raised concerns of a derailing of talks to ease the crisis.

The Ukrainian parliament on Tuesday was due to meet to discuss concessions proposed by Yanukovych, in a highly anticipated extraordinary session that could be a make-or-break moment to resolve the standoff.

With concern growing in the West that the situation in Ukraine was spiraling out of control, the country's worst crisis since independence was also set to dominate an EU-Russia summit on Tuesday.

The protests, which began in November as a drive for EU integration after Yanukovych ditched a key deal with the bloc under Russian pressure, have now turned into an all out uprising to unseat him.

Amid a febrile atmosphere in Kiev, the Dzerkalo Tyzhnia news website reported the cabinet was preparing a decree for a state of emergency which would restrict movement on some Kiev streets.

But Foreign Minister Leonid Kozhara told reporters Monday: "Today we are not considering the introduction of a state of emergency. Today, this measure is not on the table."

Tensions remained high in Kiev after several dozen radical protesters seized control of the justice ministry late Sunday.

Justice Minister Olena Lukash, who is taking part in the negotiations with the opposition, said she would ask for the talks to be broken off if the building was not freed.

If the protesters do not vacate the building, Lukash said she would also approach Ukraine's national security council with "a demand to discuss imposing a state of emergency in this country," Lukash told Ukraine's Inter channel.

After occupying the premises for over 12 hours, the activists walked out of the building but dozens dressed in battle fatigues still blocked the front entrance, an Agence France Presse correspondent said.

Opposition leaders said in a joint statement that they were still open to negotiations "despite an attempt by the authorities to abandon the negotiations and declare a state of emergency".

The statement confirmed the justice ministry building had now been emptied of protesters, whose action had been seen as a dangerous provocation by many within the mainstream opposition.

Protesters now control much of the city center of Kiev around a hub on Independence Square, with their camp protected by barricades several meters high and guarded by activists dressed in balaclavas and armed with baseball bats.

But the rebellion has now spread well beyond Kiev, with protesters occupying regional administrations in all but one region in the west of the country which has traditionally been anti-Yanukovych.

But most worryingly for the president, protests have swept to the country's east and center, usually considered more his heartland.

Protesters have now blockaded or attempted to blockade 14 of the 25 regional administrations across Ukraine.

However the security forces appear to have been fighting back in the east, using force to disperse protests in the regional centers of Dnipropetrovsk, Cherkasy and Sumy and arresting dozens, local media reports said.

Under unprecedented pressure, Yanukovych on Saturday offered the opposition posts in government including that of prime minister, but his opponents said the offer fell short of their needs.

Yanukovych proposed sharing leadership with Yatsenyuk and Klitschko as deputy prime minister.

Klitschko, who is believed to have a personal rivalry with Yatsenyuk, condemned the proposal as a "poisoned offer" in an interview with German newspaper Bild am Sonntag.

Yanukovych's office has also said the president is willing to consider constitutional changes to reduce his power and return to a system according more authority to the prime minister.

A crucial day in the standoff is expected to come Tuesday when parliament meets in an extraordinary session to debate and possibly pass some of the changes offered by the president.

"Yanukovych this week will be more inclined to negotiate as he is getting weaker and the protesters stronger," political analyst Yevgen Glibovitsky wrote in a column for online newspaper Ukrainska Pravda.

The parliament session also coincides with an EU-Russia summit where President Vladimir Putin will be hosted by European Council president Herman Van Rompuy and European Commission head Jose Manuel Barroso.

The meeting -- already shortened to less than three hours -- is expected to be shadowed by tensions over Ukraine, with Moscow slamming protesters as extremists but the West worried about police violence.

In a statement on Monday, the European Union stressed that an end to human rights violations by the government "was a prerequisite for the restoration of trust".

The European Union's enlargement commissioner Stefan Fuele was returning to Ukraine on Monday for more talks just three days after his last meeting with Yanukovych, the commission said.