KIEV, Ukraine — Anti-government protests in Ukraine’s capital escalated into fiery street battles with police Sunday as thousands of demonstrators hurled rocks and firebombs to set police vehicles ablaze. Dozens of officers and protesters were injured.

Police responded with stun grenades, tear gas and water cannons, but were outnumbered by the protesters. Many of the riot police held their shields over their heads to protect themselves from the projectiles thrown by demonstrators on the other side of a cordon of buses.

The violence was a sharp escalation of Ukraine’s two-month political crisis, which has brought round-the-clock protest gatherings, but had been largely peaceful.

In Washington, the White House blamed the tensions on Ukraine’s government for failing to acknowledge its people’s legitimate grievances, and is threatening sanctions if the use of violence continues.

National Security Council spokeswoman Caitlin Hayden said Ukraine is weakening the foundations of its democracy by criminalizing protests. She called on Ukraine to remove riot police from downtown Kiev and start talking to the opposition.

Opposition leader Vitali Klitschko tried to persuade demonstrators to stop their unrest Sunday, but failed and was sprayed by a fire extinguisher in the process. Klitschko later traveled to President Viktor Yanukovych’s suburban residence and said the president has agreed to negotiate.

Yanukovych said later on his website that he has tasked a working group, headed by national security council head Andriy Klyuev, to meet with opposition representatives to work out a solution to the crisis. However, it was unclear if either side was prepared for real compromise; throughout the crisis, the opposition has insisted on the government’s resignation and calling early presidential elections.