to "quit everything ... and go home."

Riot police managed to break through a second-floor window of the Trade Unions building and occupy most of its floors. The building has been used for more than two months as the headquarters of the anti-government protest movement, called EuroMaidan.

A session was set to take place there this morning in which opposition lawmakers hoped to pass amendments to the constitution that would curb the powers of the president. However, it was cancelled due to the violence outside. In the video below, taken by Mashable's correspondent on Feb. 18, shows protesters heaving tear gas into a group of police. (A live video feed is embedded below.)

The struggle began after dozens of protesters managed to break through a police cordon comprised of four largemeans to bring order,” said acting head of the Security Service of Ukraine Oleksandr Yakymenko and acting Interior Minister Vitaliy Zakharchenko.

In addition, a prominent ally of Yanukovych, oligarch Rinat Akhmetov, said in a statement posted to the website of his company, System Capital Management, that today's killings were an "impossible price for political mistakes."

"The bloodshed must end and negotiations restart immediately," Akhmetov said. "There are no circumstances that justify the use of force against the civilian population."

Several protesters received injuries as a result of hand-to-hand combat and sniper fire. One young man was shot in the eye. Bleeding profusely, he was taken to an ambulance by medical personnel. At least two police officers were severely beaten by protesters wielding metal pipes and baseball bats when they were separated