Fighting has broken out and smoke bombs been thrown inside Ukraine's parliament amid massive public protests in Kiev.
Clashes then erupted inside after the parliament's speaker was egged from an unknown assailant during a session ratifying the controversial deal.
Volodymr Lytvyn took cover behind two black umbrellas after the pelting, before smoke from two hurled bombs billowed inside the chamber.
Parliament cameras showed representatives punching, shoving and pulling each others' hair as the white haze descended.
A scramble then emerged for ownership of a giant yellow and blue national flag in the middle of the room as alarms sounded.
Remarkably, order in the notoriously fractious parliament was restored, although some disquiet could still be heard.
Deputies in parliament started fighting over a
Ukrainian flag
The deal, signed by president Victor Yanukovych last week, extends the lease on Russia's Black Sea Fleet naval base in Ukraine until 2042.
But the pro-Western opposition have damned the agreement with Russian president Dmitry Medvedev as an historic surrender of sovereignty.
Outside, riot squads attempted to quell thousands of their supporters as cries of "Death to traitors" and "Crimea is ours" filled the streets.
Efforts were complicated as a counter-rally backing the government also emerged, with pro-Yanukovych banners reading: "Ukraine and Russia: Strategic Partners".
The fleet deal marks a dramatic turnaround in the countries' ties.
The relationship became so bad under Mr Yanukovych's predecessor, Viktor Yushchenko, that Moscow refused to do business with him.