Demonstrators in Ukraine have vowed to stage round the clock protests in the capital Kiev, demanding the resignation of the government and the president.
Tents have been put in Independence Square to shelter those who will be spending nights and days at the anti-governmental rally.
Earlier riot police prevented protesters from storming into the administration building.
Riot police in Ukraine's capital Kiev used tear gas, truncheons and flash grenades to push back demonstrators who tried to enter the building on Sunday evening.
About 300-thousand Ukrainians took to the streets on Sunday in the biggest show of anger yet over President Viktor Yanukovych's refusal to sign an agreement with the European Union.
Yanukovych justified the decision by saying that Ukraine could not afford to break trade ties with Russia.
The EU agreement had been eagerly anticipated by Ukrainians who want their country of 45 (m) million people to break out of Moscow's orbit.
Opinion surveys in recent months showed about 45 percent of Ukrainians supporting closer integration with the EU and a third or less favouring closer ties with Russia.
Moscow tried to block the deal with the EU by banning some Ukrainian imports and threatening more trade sanctions.
A 2009 dispute between Kiev and Moscow on gas prices resulted in a three-week cutoff of gas to Ukraine.
The current protests began more than a week ago and they carried echoes of the 2004 Orange Revolution, when tens of thousands came to the square nightly for weeks and set up a tent camp along the main street leading to the square.
For Yanukovych, memories of the Orange Revolution are still raw.
Those protests forced the annulment of a fraud-tainted presidential election in which he was shown to have won the most votes.
A rerun of the election was ordered, and he lost to Western-leaning reformist Viktor Yushchenko.
Yanukovych was elected president five years later, narrowly defeating then-Prime Minister Yulia Tymoshenko, the leading figure of the Orange Revolution.
AP