At least 38 people choked to death on smoke or were killed when jumping out of windows after the trade union building was set on fire, according to police.
This video shows scenes from the burning building:
At least four others killed and 174 injured when the two sides battled against each other on the streets of the port city.
Odessa, a city on the Black Sea some 330 miles from the turmoil in the east, had remained largely untroubled by unrest since the February toppling of pro-Russia President Viktor Yanukovych.
According to reports, the violence began when marchers calling for Ukrainian national unity clashed with a pro-Russia group said to be comprised of fans of the local soccer team.
Police initially failed to stop the violence, according to local reports.
The local House of Trade Unions building was set on fire.
Howard Amos, a reporter in Russia, tweeted that “heavily outnumbered pro-Russians inside are shooting and throwing flash grenades.”
People inside the burning building were seen hanging from windows trying to escape the smoke.
Once fire engines and ambulances arrived, the fire in the trade union building was put out and pro-Russian protestors were dragged out by the police and treated for wounds before being loaded into police vans, Amos reported.
Tasneem Nashrulla is a reporter for BuzzFeed News and is based in New York.
Contact Tasneem Nashrulla at email@example.com.