The months-long standoff between Ukraine's strongarm pro-Russian president and opposition protesters in the streets of Kiev, the capital, has turned grisly in recent hours, with 18 deaths reported in clashes between police and the demonstrators. This gripping livestream is being broadcast from Kiev's main square:

The most recent round of intense violence apparently began when government authorities tried and failed spectacularly to regain control of Kiev's main square from the demonstrators who have camped there since late last year. Via the New York Times:

Mayhem gripped the center of the Ukrainian capital on Tuesday evening as riot police officers tried to drive two armored personnel carriers through stone-reinforced barriers in Independence Square, the focal point of more than two months of protests against President Viktor F. Yanukovych. Pelted by rocks and fireworks, the vehicles became stuck in the massive barricades outside the Khreschatyk Hotel and burst into flames, apparently trapping the security officers inside, prompting desperate rescue efforts from their colleagues.

All hell subsequently broke loose. Here's all the latest from Kiev.

Update 7:56 p.m. EST: Opposition leader Vitaly Klitschko says that he and other opposition heads have called off talks with President Yanukovych, who Klitschko says refuses to end the siege on the protesters. At least 21 people are dead, including at least eight police officers; here's an AP photo of protestors carrying a wounded police officer.

The large Trade Union building on the square, which was being used by protestors as a headquarters, has been set on fire with people inside.

Update, 6:15 p.m. EST: Vice President Joe Biden called Ukranian president Viktor Yanukovych, urging him to withdraw police and expressing "grave concern" over the situation.

Update, 5:20 p.m. EST: Police have stormed into the square again, attempting to rout the protestors. The New York Times is now reporting 13 dead, and officials have raised the toll to 18. On the livestream, amplified speeches from protest leaders are still punctuated by fireworks, small explosions, and lobbed Molotov cocktails.

Photos of the scene from AP and AFP/Getty:

An anti-government protester runs during clashes with riot police. Photo by Efrem Lukatsky via AP.

Anti-government protesters. Photo by Genya Savilov via AFP/Getty Images.

Riot police storm the square. Photo by Efrem Lukatsky via AP.

An anti-government protester finds cover. Photo by Efrem Lukatsky via AP.

Fires burn in the streets as anti-government protesters clash with police. Photo by Alexander Koerner via Getty Images.

Monuments to Kiev's founders burn. Photo by Efrem Lukatsky via AP.

The iconic square turned into a war zone as riot police moved slowly through opposition barricades from several directions, hurling stun grenades and using water cannon to clear protestors. Photo by Genya Savilov via AFP/Getty Images

Update, 2:17 p.m. EST: This Reuters video captures some of the crazy:

And here are some Associated Press images from the day's events:

Protesters apparently attempted to improve on barricades blocking the progress of government forces.

When the forces clashed, many protesters did so armed with improvised weapons.

Here, a protester armored in a motorcycle helmet lobs a stone.

Some protesters have improvised flame throwers and firearms from what appear to be flares.

The clashes have been hazardous to police and protesters alike.

It's not clear how the standoff will end now that it has entered a more dynamic, more violent stage.