Ukraine's opposition is set to hold a major rally on Tuesday to protest President Viktor Yanukovych's swift pursuit of closer ties with Russia and what it considers his growing authoritarianism.
The protest comes three weeks after Yanukovych and his Russian counterpart Dmitry Medvedev signed a deal allowing Moscow to keep a naval base in Ukraine for at least 25 more years in exchange for cheaper natural gas.
Yanukovych said the pact would save Ukraine billions of dollars and help the country recover from a deep economic crisis, but critics slammed it as a sell-out of Ukraine's national interests to its Soviet-era master.
Lawmakers from pro-Western opposition parties threw eggs and smoke bombs and brawled with Yanukovych supporters during a raucous session of parliament in which the controversial agreement was ratified.
Russian Prime Minister Vladimir Putin then further alarmed the opposition with a proposal to merge Ukraine's state-run gas firm, Naftogaz, with Russia's vastly larger state-controlled energy giant, Gazprom.
The leader of Ukraine's largest opposition party, Yulia Tymoshenko, has called a mass demonstration outside parliament at 10:00 am (0700 GMT) to defend the country's "sovereignty and territorial integrity".
A separate protest is being staged by Svoboda (Freedom), a nationalist group, at 8:00 am (0500 GMT).
Tuesday's demonstration will be a key test for Tymoshenko, a former prime minister, as she fights to retain her political relevance after losing a presidential election to Yanukovych in February.
Tymoshenko announced on Saturday that she would seek to have Yanukovych impeached over the agreement extending the lease of the Russian naval base, which is located in the Crimean Peninsula port of Sevastopol.
But her effort appears to be purely symbolic, since impeachment requires a two-thirds majority in parliament and the opposition is in the minority.
Last week, Tymoshenko accused Yanukovych of using methods of "dictatorship" and said he was failing to consult with civil society as he pushed ahead with pro-Russian policies.
Her accusations were echoed when a group of journalists at one of Ukraine's leading television channels, 1+1, declared in a letter that they were being subjected to "censorship" and pressured to follow a pro-government line.
The president says he supports freedom of the press.
Yanukovych became president in February, replacing the staunchly pro-Western Viktor Yushchenko, the hero of the 2004 Orange Revolution, in an election that was considered democratic by Western poll monitors.