Ukrainian anti-government protesters have tried to seize the energy ministry in Kiev and detained two policemen, according to the government.

Eduard Stavytsky, the energy minister, said he had confronted about 100 protesters on Saturday and warned them that Ukraine's energy system would collapse if they occupied the ministry. The protesters left the building and blocked roads outside.

The interior ministry said protesters had captured two of its officers and detained them in the Kiev city administration building which has been occupied by demonstrators for nearly two months. The policemen were later released.

The ministry added that another officer who had been injured while being seized by protesters had been released and was hospitalised in serious condition.

On Friday, Viktor Yanukovych, the president, said he would reshuffle the government and modify draconian laws against demonstrations.

But the key demand, for early parliamentary and presidential elections, does not appear to be on the table, nor is there any suggestion that the president is ready to sack his hardline prime minister, Mykola Azarov.

In central Kiev, anti-government activists who occupied another government building overnight were unimpressed with the concessions.

At the barricades near the Dynamo Kiev football stadium, protesters resumed their assault on police lines overnight, with barricades of burning tyres lit and projectiles fired at riot police using a giant catapult. The police doused the fires with water cannon but were not responding with force.

The majority of those at the barricade during the night were the hardcore of the protest movement, dressed in combat fatigues and ready for violence.

Earlier in the day, there had been a more mixed crowd at the barricade. Natalia, a 50-year-old engineer from the east of Ukraine, had arrived on Thursday, bringing helmets and medicine. "I came here to defend the future of my children. We have no future if we are ruled by this criminal," she said.

Sergiy, a 20-year-old student wearing a helmet and gas mask and wielding a wooden stick and a shield fashioned from a traffic sign, said he thought negotiating with the president was pointless: "This bastard is only playing for time, but the country has already risen up against him."

He claimed that a friend had been beaten up by men in civilian clothes after being seized from the protest, then taken to a forest and stripped naked.

There were many such stories. A video of one incident where riot police humiliated a naked protester and took trophy photographs has circulated widely, sparking further outrage.

Mikhailo Gavrilyuk told journalists he was seized on Wednesday as he attempted to help an injured fellow protester. "They dragged me off and started beating me. They threw me to the ground, put their legs on my head and photographed me. They took turns to beat me, and someone suggested they should cut off my hair, which they then did with a knife."

He later had his clothes taken away and was taken naked to a police station, he said. The interior ministry has announced an investigation into the incident, but Gavrilyuk said he expected fellow protesters to exact "terrible revenge".

The changes to the laws and government reshuffle are expected to take place at an emergency parliamentary session scheduled for next Tuesday.

But as the weekend approached, Yanukovych faced not only a standoff in Kiev but also a breakdown in his authority across the west of the country, where activists have seized administrative buildings in a number of cities and built barricades around them.

The interior ministry claimed that a policeman had been shot while walking home, unarmed, on Friday night. Details of the incident, and whether it was linked to the protest, were not forthcoming.

The trio of opposition political leaders who took part in discussions with Yanukovych have come under pressure to take a firmer stance against the government and withdraw from negotiations.

The former heavyweight boxer Vitali Klitschko on Wednesday promised to "go on the attack" if Yanukovych did not call elections within 24 hours, but is now calling on protesters to hold a ceasefire, uneasy about being seen as responsible for escalating the situation. However, he said on Friday that the protest would not wane without Yanukovych's resignation.

• This article was amended on 27 January 2014. The original contained an editing error that said Kiev's Olympic stadium was the ground of the Dynamo Kiev football team. This has been corrected to say Dynamo Kiev football stadium.