Ukraine's embattled president, Viktor Yanukovych, on Saturday night made a surprising and wide-ranging compromise offer to the protesters who have occupied his capital, promising to make an opposition leader prime minister, give amnesty to those involved in clashes with police and institute major constitutional reforms. The trio of politicians who have become the de facto leaders of the protests rejected the offer but said they were willing to negotiate.

After several hours of discussions on Saturday, it was announced that Yanukovych had offered the prime minister's job to Arseny Yatsenyuk, of jailed former prime minister Yulia Tymoshenko's Fatherland party. He also offered a deputy prime minister post to Vitali Klitschko, a former heavyweight boxer, and promised public debates with him.

"The president is confident that joint work with the opposition will help Ukraine unite and conduct reforms necessary for the state and the society," said a statement on Yanukovych's website, which also demanded that the street protests end.

Klitschko, however, said protesters would not leave Independence Square and the surrounding barricades. Yatsenyuk wrote on Twitter: "No deal Yanukovych, we're finishing what we started. The people decide our leaders, not you."

But when Yatsenyuk addressed the crowds in Independence Square he did not rule out taking the post and said that an emergency parliamentary session called for Tuesday would be key. Klitschko said he would only agree on a deal when Yanukovych agreed to call presidential elections.

The negotiations were the culmination of a week when everything changed in Kiev. Protests that were previously peaceful turned violent, with demonstrators throwing molotov cocktails and firing projectiles from hastily assembled catapults, while opposition leaders who had demanded negotiations began to speak about taking a "bullet in the forehead" in the struggle for the country's future. What for two months has been an edgy but largely bloodless standoff in Kiev's icy streets suddenly became red hot, with violence on both sides, at least three deaths and hundreds of injuries.

Earlier in the day, the interior minister said that force could be used to disperse the protests. "The events of the last days in the capital have shown that our attempts to solve the conflict peacefully, without recourse to a confrontation of force, remain futile," said Vitali Zakharchenko. "Our calls have not been heeded and a truce is being violated."

Over the past week, there has been a complete breakdown of understanding between protesters and police in Kiev, with the violence aggravating an already tense situation. Andrei Kiselev, a Russian videojournalist, recounts a chilling tale of being abducted, one of many similar stories. He was filming a group of activists who were travelling by bus – searching for a group of fellow protesters who said they had been kidnapped by police – when the bus was forced off the road by a police bus and the activists seized. He said the group were all beaten with truncheons, and Kiselev was not spared despite screaming that he was a journalist and offering to show his press pass.

He suffered a broken nose, black eye and large swellings all over his body. He also lost £3,000 of camera equipment, but worse was to come. He and the activists were loaded into the police bus and told to lie on the floor, he recounted from the Kiev apartment where he is recuperating. "They put their legs on our heads and if anyone tried to move they beat us. We had no idea where we were going, and many people were in extreme pain, as we had bad injuries. Eventually they made us get out of the bus, after we had been driven for half an hour, and we were forced to kneel in the snow for an hour, while they searched us."

Eventually, they were loaded back on to the bus, and told they would be "thrown under the ice" and killed, but instead were taken to a police station. By this point, worried colleagues in Moscow and Kiev had raised the alarm online, and the police sheepishly freed Kiselev, although none of his equipment was returned, nor his wallet. He says he does not know what happened to the protesters who were detained with him.

Others have even more harrowing tales. Igor Lutsenko, a journalist and activist, says he was kidnapped from Oleksandrivska hospital with another activist, Yuri Verbytsky, on Tuesday.

Lutsenko had taken Verbytsky to hospital for treatment to an injury suffered in clashes with police, when a group of men in civilian clothes dragged them into a minivan and drove them to a forest outside town, where they interrogated and tortured, Lutsenko told the Observer by telephone from his hospital bed. He is convinced the men who abducted them were working for the authorities.

"They behaved during the interrogation like people who have been doing this for many years," he said. "I don't think they were targeting me specifically. I think it was an act of intimidation, to let people know that this kind of thing could happen to anyone."

After a severe beating, Lutsenko was able to crawl out of the forest and make it to hospital. Verbytsky was not so lucky. He was found dead in Boryspil forest, just outside Kiev, on Wednesday. His body was found with his hands tied behind his back and a bag on his head. Police say he died of hypothermia. There appears considerable evidence to suggest that it is not only shadowy plainclothes thugs engaged in the violence.

One of the defining images of the week came in a video that received millions of hits on YouTube, of activist Mikhailo Gavrilyak stripped naked and being forced to walk in the freezing snow by a group of riot police, who are seen slapping the back of his head and taking photographs. Gavrilyak, who was later released, said he was subjected to repeated beatings and had his hair cut off with a knife. He said protesters would exact a "terrible revenge" for his ordeal.

Vitali Klitschko, an opposition leader, released a statement calling on officials and police to disobey orders: "Do not carry out criminal orders, do not approve unjust and illegal decisions, do not participate in repression against the citizens of Ukraine, our own people. Protect them today!"

But while there have been reports of some police defecting in the west of the country – which has never supported Yanukovych – in Kiev there is said to be anger and a desire for revenge among riot police, who have spent countless days standing in freezing temperatures, sheltering from attack, and in some cases watching their colleagues being hit by molotov cocktails. The video of Gavrilyak's humiliation was not posted by a conscience-stricken whistleblower, but was discovered on the social network page of a riot police officer, with the caption: "We're going to fuck you up."

The stories of police brutality have only intensified feelings on the barricades. A truce called on Thursday has broken down, and activists continued to burn tyres and shoot projectiles at police. The storming of administration buildings in the west of the country continued, with activists occupying buildings in 10 cities as the situation continues to slip further out of Yanukovych's control.

The opposition leaders said Yanukovych seemed "completely changed" on Saturday as compared with negotiations in the week, and appeared worried by the situation in the west. His offer to give top posts to the opposition as well as make constitutional changes is much further than he has gone before, but with tensions so high it remains unclear whether anything other than Yanukovych's resignation will be enough to end the standoff on the streets.

At midnight, hundreds of protesters smashed their way into a building housing interior ministry troops, wielding clubs and throwing molotov cocktails. The protesters demanded the troops leave the building, which they eventually did after a tense standoff lasting four hours. The retreat was negotiated by Klitschko and the troops left the building as the violence subsided.