As riots spread from Ukraine’s embattled capital to nearly half of the country, President Viktor Yanukovych promised on Saturday to reshuffle his government and make other concessions, but a top opposition leader said nothing short of his resignation would do.

After nearly two months of ignoring mass demonstrations calling for his ouster, Mr. Yanukovych offered to meet some of their demands, after crowds angered by the deaths of at least two protesters and allegations of abuse by authorities besieged government buildings in scores of cities in western Ukraine.

At a meeting today with religious leaders, Mr. Yanukovych vowed that, at a special parliament meeting on Tuesday, he would push through changes to his Cabinet, grant amnesty to dozens of jailed activists and amend harsh anti-protest legislation.

But Vitali Klitshko, an opposition leader who is a world heavyweight boxing champion, declared on Saturday that the only way to end the street protests, known as the Maidan after the central Kiev square occupied by demonstrators, is for Mr. Yanukovych to resign.

“Just a month ago, the Maidan would have gone home,” Mr. Klitschko told reporters tonight, according to the Interfax news agency. “Today, people are demanding the president’s resignation.”

The protest law enacted last week appeared to have backfired on Mr. Yanukovych, sparking confrontations in which demonstrators threw stones and firebombs at police, who responded with tear gas and rubber bullets.

The violence since Sunday was a harsh contrast to the determined peacefulness of the anti-government protests that have gripped the country for the last two months.

The rallies broke out after Mr. Yanukovych scrapped a key treaty with the European Union in order to secure a bailout loan from Russia. President Vladimir Putin had pressed hard to keep Ukraine in his nation’s political and economic orbit, but more Ukrainians favour closer ties with the 28-nation EU than a new alliance with Russia.

At least two demonstrators were killed this week in clashes with police and protesters have seized government offices in cities in western Ukraine, where support for Mr. Yanukovych is thin.

In a separate incident, a protester was found dead outside Kiev this week after going missing from a hospital together with a prominent activist who was beaten but survived.

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