BBC News - Ukraine protest: Police clash with pro-EU crowd in Kiev Ukrainian police have used tear gas in fresh clashes with pro-EU activists in the heart of Ukraine's capital Kiev. Reports say some protesters tried to enter the government building but were pushed back by police. 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. The activists included supporters of jailed opposition leader Yulia Tymoshenko, who urged the government to release her - a plea that has also been made by EU politicians. More than 100,000 people rallied in Kiev on Sunday. Hundreds of protesters stayed in tents on the 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. The Interfax-Ukraine new agency said several hundred protesters gathered outside the government building on Monday and various objects "started flying in the direction of the police," prompting the police to don gas masks and form a human chain. Some activists told a BBC reporter at the scene that there were athletic-looking young people in the crowd whom they suspected of being agents provocateurs. In addition to Vitali Klitschko, opposition leaders Arseniy Yatsenyuk and Oleh Tyahnybok addressed the crowd. They urged President Yanukovych to sack Prime Minister Mykola Azarov and to sign the association agreement with the EU. The signing had been set for 29 November in the Lithuanian capital Vilnius, where several other ex-Soviet states will forge closer ties with the EU.