Unrest across Ukraine turned violent Friday in key eastern and southern cities. It was the deadliest day since 100 people were killed in Kiev in February: At least 73 people were killed in violence around the country.

An estimated 31 people were killed when a building occupied by pro-Russian protesters was set on fire by pro-Ukrainians in the southern city of Odessa. The pro-Russian protesters retreated to the building and pro-Ukrainian protesters hurled Molotov cocktails into the windows to set it ablaze.

People were seen scrambling out of windows to escape, while others jumped to their death. Some pro-Ukrainian protesters brought in ladders and ropes to save those stranded in the building. Some of the pro-Russian protesters who did escape alive were arrested by police, to chants of “Odessa, Ukraine,” from pro-Ukrainians.

Journalist Howard Amos criticized the police there for failing to stop violence “even when it was clear people were getting killed.”

Reports say the violence started between rival protest camps in Odessa. Riot police attempted to hold them back with tear gas and a police line but eventually retreated to safety. Amos says “well-prepared but amateur extremists” from both camps spurred the violence. Four people were killed in the clashes.

This video was taken from the side of pro-Russian protesters, who can be seen breaking through police lines toward their pro-Ukrainian rivals:

Russia blames the Ukrainian government and extremist nationalists from the Right Sector party for the violence.

Meanwhile, the Ukrainian military has surged forward in its campaign to retake control of embattled regions in eastern Ukraine. Pro-Russian separatists shot down two Mi-24 Hind helicopters, killing two airmen and wounding at least one other. One helicopter was shot down by a surface-to-air missile, which the Ukrainian government says proves covert Russian military intervention in eastern Ukraine. Video footage has surfaced of separatists with what appears to be a wounded Ukrainian airman.

According to Minister of Interior Arsen Avakov, the Ukrainian military has surrounded and secured nine checkpoints around the eastern city of Slovyansk, a hotbed of pro-Russian separatist activity. The Interior Ministry confirmed that two Ukrainian soldiers were killed in the fighting.

Separatists have vowed to continue the fight:

Russian President Vladimir Putin said the Ukrainian campaign “effectively destroyed the last hope for the implementation of the Geneva agreements” meant to reduce tensions in the country. He told German Chancellor Angela Merkel that Ukraine must pull its military out of southeast Ukraine, but Kiev has no plans to leave the east.