Ukrainian police have used tear gas in fresh clashes with pro-EU activists in the heart of Ukraine's capital Kiev, BBC reported.Reports say some protesters tried to enter the government building but were pushed back by police with tear gas.Hundreds of protesters gathered on European Square again after a huge rally on Sunday - the biggest since the 2004 Orange Revolution.They are angry at the government's decision not to sign a major trade and association deal with the EU.In recent weeks the government has come under Russian pressure not to sign the pact, but to join a Russian-led customs union with former Soviet states instead.Opposition leaders, including world heavyweight boxing champion Vitali Klitschko, called the government's decision "shameful" and vowed to keep up mass protests in central Kiev indefinitely.More than 100,000 rallied in Kiev on Sunday and some protesters stayed in tents on the main square overnight, despite an order from the authorities not to do so.Tents were a big feature of the pro-Western Orange Revolution, which challenged Russia's traditional influence in Ukrainian politics.Viktor Yanukovych is president now, having been elected in 2010, but back in 2004 he was toppled by the Orange Revolution after an election widely condemned as fraudulent.The authorities have launched criminal proceedings against some activists who clashed with police on Sunday. ANN.Az