— A war of nerves, stones, molotov cocktails and a full-size catapult, punctuated by stun grenades, reset the calculations of Ukrainian politicians Monday and seemed to be moving the country closer to a final showdown.

Helmeted protesters and riot police spent the day skirmishing across a no-man’s land where they had fought in earnest the night before, and as darkness fell again Monday both sides prepared to push for an advantage, even as the city all around them went about its business.

The young men who on Sunday evening had moved the focus of the two-month-old protest from the campsite at Independence Square to the battleground on nearby Hrushevsky Street were elated by the conflict and by their ability to stand up to the Berkut riot police who confronted them.

But there was no doubt that one side or the other is likely to put an end to the Hrushevsky Street standoff — and probably soon.

The opposition politicians who have been leading the protest have objected strenuously to a violent turn of events driven by nationalist groups looking for a fight with the authorities. But other demonstrators grudgingly applauded the escalation as the only possible step after the government pushed through draconian laws restricting freedom of speech and assembly late last week.

President Viktor Yanukovych, the focus of the demonstrators’ anger, announced that his government would enter into talks with the opposition to find a way out of the crisis. But he said he wouldn’t take part in them. Leaders of the opposition parties appointed deputies to stand in for them. Deep into the evening, the talks finally started.

The protest began in November when Yanukovych backed off signing a trade agreement with the European Union, and it had recently been winding down out of exhaustion and frustration. But Yanukovych reignited it with the new legislation, which the opposition called a coup, and furious crowds filled the streets once more.

Yanukovych addressed the nation Monday evening, urging dialogue and vowing to maintain peace.

“I believe in the people of Ukraine. I am sure of the wisdom of our people,” he said, according to the Interfax news service.

“And I stand ready to serve the state and people faithfully and honestly for as long as I am strong enough to do this and the people have confidence in me.”

He warned that violence is a threat to the future of the nation.

The leader of the UDAR party, the boxer Vitali Klitschko, made an urgent appeal Monday to Ukrainians to travel to Kiev, bringing their cars and trucks to form more blockades. This was echoed later in the day by a protest organizer, Andrei Parubiy, who said he feared that a police attack on the Maidan, or Independence Square, was imminent.

Kiev health officials said that 103 protesters were treated and 42 of them hospitalized late Sunday and early Monday. About 100 police officers were injured, the Interior Ministry said, and 61 of them hospitalized.

Police said 31 people have been detained. Two correspondents for U.S.-financed Radio Liberty were detained Monday while they were covering the protests but were later released, Radio Liberty reported. They said they had been beaten. Media officials in Kiev said at least 15 journalists had been hurt while covering the violence Sunday night and Monday.

By Monday afternoon Hru­shevsky Street had become something of a spectacle. As supporters banged away on sheets of metal and hit street signs and lampposts over and over again with hammers, the percussive din filling the smoky air all day long, daredevils would break out of the crowd and run up to a barricade of burned-out buses, throwing paving stones at the solid police lines, about 30 yards back behind the buses and protected by a wall of shields.

Whenever the crowd seemed to be inching up behind them, the police would toss stun grenades that emitted an irritant in their smoke, and the people would move back.

At one point a squad of Interior Ministry troops also moved up toward the buses and, surprisingly, began throwing stones back at the protesters. One group of protesters, meanwhile, had spent the day constructing a catapult, about eight feet high, which began heaving rocky missiles about 9 p.m..

The back-and-forth drew hundreds of onlookers, cheering as if at a soccer match. In the early evening several protesters threw molotov cocktails down from a ceremonial arch at the entryway to Dynamo Stadium, most of them falling harmlessly into a small park but eliciting cheers from the crowd.

In Moscow, Russian officials have accused the United States of stirring up trouble, and on Monday Leonid Slutsky, the head of Russia’s parliamentary committee on Eurasian integration, accused “strategic friends” from the West of inflaming events in Ukraine.

“They finance them via various nongovernmental organizations, which start problems that escalate into disturbances and violence not in line with civilized European ideas, to which Ukraine is being persistently urged,” Slutsky was quoted by the Interfax news agency.

In Kiev, a member of parliament from the ruling Party of Regions, Vadym Kolesnichenko, made the same accusation.

European Union officials have expressed concern about events in Ukraine but said Monday that they will not consider sanctions against Yanukovych or his allies.

On Twitter on Monday, an opposition-minded young woman named Kateryna Kruk put it this way:

“EU is deeply concerned. I’m sure this thought’ll warm me up while facing riot police or going to jail for tweeting. Then I’ll be concerned too.”

Lally reported from Moscow.