KIEV, Ukraine — Thick black smoke from burning tires is engulfing the downtown Ukrainian capital as an ultimatum issued by the opposition to the president to call early elections or face street rage was set to expire with no sign of a compromise.

The three main opposition leaders urged protesters late Wednesday to refrain from violence for 24 hours until their ultimatum to President Viktor Yanukovych expires.

The largely peaceful protest against Yanukovych's decision to shun the EU and turn toward Moscow in November descended into violence Sunday when demonstrators marched on official buildings.

For days protesters hurled fire bombs and stones at police, which retaliated with stun grenades, tear gas and rubber bullets. The deaths of two protesters Wednesday fueled fears of more violence.