KIEV, Ukraine — A deal aimed at ending a lethal spiral of violence in Ukraine began to show serious strains late Friday just hours after it had been signed, with angry protesters shouting down opposition members of Parliament who negotiated the accord and a militant leader threatening armed attacks if President Viktor F. Yanukovych did not step down by morning.

Russia, which joined France, Germany and Poland in mediating the settlement, introduced a further element of uncertainty by declining to sign the accord, which reduces the power of Mr. Yanukovych, an ally of Moscow. This stirred fears that Moscow might now work to undo the deal through economic and other pressures, as it did last year to subvert a proposed trade deal between Ukraine and the European Union. But American officials said Mr. Putin told Mr. Obama in a telephone call on Friday that he would work toward resolving the crisis.

The developments cast a shadow over a hard-fought accord that mandates early presidential elections by December, a swift return to a 2004 Constitution that sharply limited the president’s powers and the establishment within 10 days of a “government of national trust.”

In a series of votes that followed the accord and reflected Parliament’s determination to make the settlement work, lawmakers moved to free Mr. Yanukovych’s imprisoned rival, former Prime Minister Yulia V. Tymoshenko; grant blanket amnesty to all antigovernment protesters; and provide financial aid to the hundreds of wounded and families of the dead.