Ukrainian police have fired teargas and used batons on protesters after parliament voted to make Russian the main language in schools and local government in some parts of the country.
Opposition politicians had led the demonstration outside a building where President Yanukovich was due to speak, after the bill was rushed through giving opponents little time to cast their vote.
World boxing champion Vitaly Klitschko, who founded his own opposition Udar (“Blow”) party, was slightly injured in the scuffles.
“Today people lost all belief in the rule of law in Ukraine,” he said. “The events in parliament prompted thousands to come here and call on the president as the guardian of the constitution. And we all wonder what answer he’ll give.”
Opponents have urged President Viktor Yanukovich to veto the bill. They see it as a threat to Ukrainian identity and believe the ruling Party of Regions is trying to curry favour with disenchanted voters in Russian-speaking regions ahead of parliamentary elections.
It denies the claim.
“It’s clear that the solution to this question was delayed and and blocked over several years. This question was included in all official programmes of our political party, so we thought it necessary to vote when it was voted,” said Olexandr Yefremov of the Party of Regions.
The interior ministry said protesters had assaulted police. A court banned some demonstrations.
Opponents have the backing of the parliamentary speaker who tendered his resignation over the bill.
But it is likely to be popular in eastern Ukraine – and in Moscow.