KIEV: Pro-EU Ukrainian demonstrators on Monday kept up their protest against President Viktor Yanukovych after symbolically toppling the statue of the Soviet Union's founder Vladimir Lenin during a gigantic rally in central Kiev.

Hundreds of protestors braved early morning sub-freezing temperatures to maintain the open-ended demonstration on Independence Square in Kiev while others staffed barricades thrown up the day earlier around key government buildings.

“It is impossible now to make a step backwards,” said protestor Volodymyr Kiblyk from the central town of Znamenka who has been at the Kiev protests for two weeks.

Hundreds of thousands had on Sunday filled Independence Square to bursting point to denounce Yanukovych's rejection of an EU pact under Kremlin pressure, in the biggest protests since the 2004 Orange Revolution.

In a hugely symbolic denouement to the rally, dozens of masked protesters, some brandishing flags of the ultra-nationalist Svoboda (Freedom) party, tore down a 3.4 metre (11 feet) high statue of Lenin after putting a rope noose round his neck.

They then hacked away with axes at the remnants of the monument lying flat on the ground. Parts of the statue including one of its hands were afterwards triumphantly brandished at the main demonstration on Independence Square.

Police opened a criminal probe into “mass riots” over the felling of the monument but said Monday that so far no-one has been arrested over the incident.

'The whole of Kiev is a Maidan'

Upping the stakes in the confrontation, demonstrators have erected one-and-a-half metre (five feet) high barricades outside the seat of government, making it impossible for ministers to go to their offices by car.

Jailed ex-premier Yulia Tymoshenko told Sunday's rally the opposition was demanding the “immediate” resignation of Yanukovych, in a no-holds-barred statement read by her daughter.

United Nations Secretary-General Ban Ki-moon called Yanukovych by phone on Sunday to urge for dialogue with his rivals as large-scale protests gripped Kiev, the UN said.

European Union foreign policy chief Catherine Ashton will travel to Ukraine this week to find a way out of the crisis, the EU Commission said. The Interfax news agency said her visit would take place Tuesday.

Ukraine's Security Service (SBU) said it had opened an investigation into alleged attempts by politicians to seize power, in an apparent bid by the state to target key opposition figures.

The size of the protest, the third mass rally in successive weekends, increased the pressure on Yanukovych who further galvanised his opponents by meeting Russian President Vladimir Putin in almost total secrecy on Friday.

The opposition is calling another major rally for Monday in a bid to sustain the momentum and force concessions from Yanukovych.

“The whole of Kiev is Maidan now,” said opposition leader Arseniy Yatsenyuk, referring to Independence Square by its local name. He claimed the Yanukovych administration planned to impose a state of emergency.

The protests in Ukraine have raged for over two weeks after the government announced it was halting the work on political and free trade agreements with the European Union.

Protesters have seized control of Independence Square for over a week, setting up a tent city. The protesters have also occupied Kiev city hall with dozens again sleeping there overnight on Sunday.

The opposition threatened to also blockade Yanukovych's luxurious Mezhygirya residence on the banks of the Dnipro River outside Kiev if he did not dismiss the government within the next 48 hours.

The president on Friday incensed protesters further by discussing a strategic partnership treaty with Putin, who wants Ukraine to join a Moscow-led Customs Union.

Analysts believe Russia may have offered Ukraine cheaper gas and billions of dollars in aid in exchange for joining the Customs Union at Yanukovych's closed-door meeting with Putin on Friday in Sochi.