Kiev, Reuters—Police clashed with protesters who blockaded a building in central Kiev on Sunday and the fate of Ukraine’s government was up in the air after embattled President Viktor Yanukovych offered opposition leaders key posts.

One of the president’s main foes called his offer a “poisoned” attempt to kill off a protest movement in a country plunged into political unrest by Yanukovych’s U-turn away from the European Union and toward Russia.

In the latest violence, a few thousand protesters tried to storm an ornate cultural center where hundreds of security forces were gathered in central Kiev, a few hundred meters from the hub of weeks of opposition protests on Independence Square.

Demonstrators threw stones and smoke bombs while police fired stun grenades and sprayed water into the crowd.

The police and security forces later left the building, its windows shattered, and streamed out through a corridor created by the crowd after an opposition leader, Vitaly Klitschko, arrived at the scene and helped negotiate a solution.

The two-hour, pre-dawn confrontation came after Yanukovych made his biggest concession yet in a two-month-old standoff that has cast Ukraine into crisis, killed at least three people and deepened tension between Russia and the West.

Yanukovych abruptly abandoned plans to sign political association and free trade deals with the European Union in November, pledging instead to improve ties with former Soviet master Russia and angering millions who dream of being in Europe.

Hoping to end protests that threaten to bring the country to a standstill, Yanukovych on Saturday offered former economy minister Arseny Yatsenyuk the post of prime minister. Klitschko, a former international boxing champion, was offered the post of deputy prime minister responsible for humanitarian issues, a statement on the presidential website said.

The presidency linked its offer to the opposition reining in violent protesters. Though the protest movement is largely peaceful, a hard core of radicals have been fighting pitched battles with police away from Independence Square.

Opposition leaders, whose power base is among protesters massing in the square whose name evokes the independence Ukraine gained in 1991, continued to press for concessions including early elections and the repeal of an anti-protest law.

“We are ready to take on this responsibility and take the country into the European Union,” Yatsenyuk was quoted as telling crowds on Independence Square after emerging from talks with Yanukovych. But he added this would entail the freeing of former prime minister Yulia Tymoshenko, who was jailed in 2011.

Klitschko told German newspaper Bild am Sonntag: “This was a poisoned offer by Yanukovych to divide our protest movement. We will keep on negotiating and continue to demand early elections. The protest by Ukrainians against the corrupt president must not have been in vain.”

Opposition leaders say Yanukovych has betrayed Ukraine and are calling for a presidential election long before the next one is due in spring 2015. Klitschko said it must be held this year.

The United States has warned Yanukovych that failure to ease the standoff could have “consequences” for its relationship with Ukraine. Germany, France and other Western governments have also urged him to talk to the opposition.

Russia on Saturday stepped up its warnings against international interference in Ukraine, telling European Union officials to prevent outside meddling and cautioning the United States against inflammatory statements.