KIEV, Ukraine—Weeks of stalemate between the government and protesters here were shattered Tuesday by a burst of deadly violence that provoked a massive crackdown.

Authorities stormed the activists' encampments in the center of the capital, leading to fiery battles that lasted into the night.

At least 18 people were killed, and the number of injured climbed into the hundreds as police advanced into Kiev's Independence Square, the heart of the protest movement, with the flash and thud of concussion grenades. As fighting progressed, so did the weaponry; many of the injured arrived at makeshift hospitals with gunshot wounds.

At a hastily assembled medical center near the gold-domed St. Michael's cathedral above the square, doctors set up tables to treat the more seriously injured protesters.

Early Wednesday, the police were in control of almost half of the square and several floors of a trade union building, used as an anti-government headquarters, were on fire, according to Reuters. The government said seven police officers and 11 protesters had died in the violence, the Associated Press reported.