KIEV, Ukraine - Opposition leaders in Ukraine rejected President Viktor Yanukovych's offer of talks Wednesday, saying they will not sit down with him until he fires his government and releases all arrested demonstrators.

That stance reflected their growing confidence after the abrupt withdrawal of riot police from parts of Ukraine's capital early Wednesday raised protesters' hopes that weeks of demonstrations have eroded police support for Yanukovych and his government.

Yanukovych issued an invitation late in the day to political, religious, and civil figures to join a national dialogue. But it gave no details about a proposed date for the talks - and could have been merely an attempt to buy time and mollify Western officials.

The opposition reaction was scathing.

"Instead of a roundtable, what we got is a breakup [with] truncheons," opposition leader Oleksandr Turchynov said. "The authorities are driving into a dead end."

Yuri Lutsenko, a former Interior Minister who is now another opposition leader, said the police retreat shows "basically only some units remain" loyal to Yanukovych.

"This is a great victory," Arseniy Yatsenyuk, leader of the largest opposition party in parliament, said of the police withdrawal. He spoke from the stage at Kiev's central Independence Square, where protesters have set up an extensive, round-the-clock protest tent camp.

Western diplomats have increased their pressure on Yanukovych to seek a solution to the tensions that have paralyzed this economically troubled nation of 46 million. In response, Prime Minister Mykola Azarov and other officials promised Wednesday that police would not act against peaceful protesters.

"I want to calm everyone down - there will be no dispersal" of protesters, Interior Minister Vitaly Zakharchenko said in a statement, which did not explain why thousands of helmeted and shield-bearing police were deployed in the first place.

Assistant U.S. Secretary of State Victoria Nuland met with Yanukovych on Wednesday after visiting the protest camp.

"I made it absolutely clear that what happened last night, what is happening in security terms here, is absolutely impermissible in a European state, a democratic state," she said, referring to police scuffles with protesters.

In Washington, the State Department said it was evaluating all options, including possible sanctions against Ukraine. It did not provide details.

Yanukovych's shelving in November of an agreement with the European Union to deepen economic and political ties has set off weeks of protests.