KIEV, Ukraine: About 50,000 demonstrators rallied in the center of Kiev on Sunday to demand that the Ukrainian government reverse course and sign a landmark agreement with the European Union in defiance of Russia.

The protest was the biggest Ukraine has seen since the peaceful 2004 Orange Revolution, which overturned a fraudulent presidential election result and brought a Western-leaning government to power.

The rally was led by Ukraine’s top opposition figures, who called for the protests to continue until President Viktor Yanukovych agreed to sign the free trade and political association deal with the EU at a summit on Friday.

Sunday’s demonstration was sparked by anger over the government’s sudden move last week to pull out of the EU agreement and focus instead on trade ties with Russia, under strong pressure from Moscow.

After the rally at European Square, dozens of protesters clashed with police near the government building, while another standoff took place outside Yanukvoych’s office. Police said they used tear gas at the government building after protesters threw a smoke bomb and refused to back off.