AP, Ukraine : Thick black smoke from burning tires engulfed parts of downtown Kiev as an ultimatum issued by the opposition to the president to call early election or face street rage was set to expire with no sign of a compromise on Thursday. The three main opposition leaders urged protesters late Wednesday to refrain from violence for 24 hours until their ultimatum to President Viktor Yanukovych expires. They demanded that Yanukovych dismiss the government, call early elections and scrap harsh anti-protest legislation that triggered the violence. The largely peaceful protest against Yanukovych’s decision to shun the EU and turn toward Moscow in November descended into violence Sunday when demonstrators, angered by the passage of repressive laws intended to stifle the protest, 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 Wednesday deaths of two protesters – the first fatalities in more than two months of protests – fueled fears of more violence. Police on Wednesday tore down barricades and chased the protesters down the hill from official buildings, but demonstrators later set hundreds of tires ablaze and regained their positions under plumes of heavy smoke helped by the wind blowing in the police direction. Tensions were high Thursday, amid reports of abduction of activists by police, but police and demonstrators, separated by the wall of smoke, refrained from any decisive action. The three main opposition leaders, who addressed the crowds on Kiev’s main Independence Square after meeting the president on Wednesday vowed to lead the demonstrators in battles with police if their demands are not met. If Yanukovych doesn’t concede, “tomorrow we will go forward together. And if it’s a bullet in the forehead, then it’s a bullet in the forehead, but in an honest, fair and brave way,” declared one of them, Arseniy Yatsenyuk. On Wednesday, riot police beat and shot at protesters, volunteer medics and journalists. The Interior Ministry said 70 protesters were arrested. The United States responded by revoking the visas of Ukrainian officials linked to violence and threatened more sanctions. But it also condemned the extreme-right radical protesters for their aggressive actions. The EU condemned the violence and said it was also considering action against the Ukrainian government.