KIEV, Ukraine -- A top Ukrainian opposition leader on Thursday urged protesters to maintain a shaky cease-fire with police after at least two demonstrators were killed in clashes this week, but some in the crowd appeared defiant, jeering and chanting "revolution" and "shame."
Emerging from hours-long talks with President Viktor Yanukovych, opposition leader Oleh Tyahnybok asked demonstrators in Kiev for several more days of a truce, saying the president has agreed to ensure the release of dozens of detained protesters and stop further detentions.
But other opposition leaders offered mixed reports on the meeting outcome, with Vitali Klitschko saying negotiations had achieved little. He and Mr. Tyanhnybok were booed at the barricades by angry demonstrators, and the atmosphere appeared tense.
"We are not going to sit and wait for nobody-knows-what," said Andriy Pilkevich, a ski mask-wearing protester who was building barricades near police lines from giant bags of ice. "Those who want to win must fight."
Interior Minister Vitali Zakharchenko issued a statement guaranteeing that police would not take action against the large protest camp on Independence Square, known as the Maidan. He also called on police to exercise calm and not react to provocations.
The developments came as hundreds of enraged protesters in several regions in western Ukraine, where Mr. Yanukovych has little support, seized government offices and forced one governor loyal to the president to resign.
At least two people were killed by gunfire Wednesday at the site of clashes in Kiev. Demonstrators had pelted riot police with barrages of stones and set police buses on fire, while officers responded with rubber bullets, tear gas and stun grenades.
Opposition leaders had set a Thursday evening deadline for the government to make concessions or face renewed clashes. Protesters had quenched barricades that had been set on fire, but lit them again during the evening.
Although one opposition leader, Arseniy Yatsenyuk, said after the talks that "there is a really good chance" of stopping the bloodshed, Mr. Klitschko was more downbeat. "The only thing we were able to achieve was not much," he told the crowd. He urged protesters to refrain from violence and continue peaceful protests to avoid further bloodshed.
"I am afraid, yes, I am afraid of human losses," Mr. Klitschko said. "We will be widening the territory of the Maidan further until these guys start reckoning with us."
The president called a special session of Parliament next week to discuss the tensions, telling the Parliament speaker: "The situation demands an urgent settlement." But there was no indication that the move represented a compromise, since the president's backers hold a majority of seats.
The protests began two months ago after Mr. Yanukovych turned away from closer ties with the European Union in favor of getting a bailout loan from Russia. They turned violent this week after he pushed through harsh anti-protest laws, rejecting demonstrators' demands that he resign and call new elections.
On Thursday, the violence spread to western Ukraine, where most residents want closer ties to the 28-nation EU.
In Lviv, near the Polish border some 280 miles west of Kiev, hundreds of activists burst into the office of the regional governor, Oleh Salo, a Yanukovych appointee, shouting "Revolution!" After surrounding Mr. Salo and forcing him to sign a resignation letter, an activist ripped it out of his hands and lifted it up to cheers and applause from the crowd. Mr. Salo later retracted his signature, saying he had been coerced.
Meanwhile, hundreds of protesters smashed windows, broke doors and stormed into the governor's office in the city of Rivne, shouting "Down with the gang!" -- a common reference to the Yanukovych government. Once inside, they sang the national anthem.
Angry crowds also besieged government offices in other western regions.
Meanwhile, anger spread after a video was released online appearing to show police abusing and humiliating a naked protester at what looked to be a site close to the Kiev clashes. In the video, a young man wearing nothing but socks, his body covered with multiple bruises, was forced to stand in the snow in freezing temperatures as a policeman punched him in the head, and others forced him to pose for photos.
The Interior Ministry issued a statement apologizing "for the impermissible actions of people wearing police uniforms," and said it had launched an investigation into the incident.
The protests have been centered on Kiev's main square, where demonstrators have defended a large tent camp for nearly two months. On Wednesday, riot police moved to dismantle barricades erected next to a government district, and two people were fatally shot in the clashes.
The opposition has blamed the deaths on authorities, but Prime Minister Mykola Azarov said Thursday that the two men's wounds were caused by hunting rifles, which police do not possess. The Interior Ministry said the men could have been killed to escalate the crisis.
The opposition maintains that as many as five people died in Wednesday's clashes, though they say they have no evidence because the bodies were removed by authorities.