Hundreds of protesters have clashed with riot police in central Kiev and Ukraine's parliament speaker offered to resign amid uproar after a move to boost the status of the Russian language in the former Soviet republic.
Riot police on Wednesday fired tear gas and used batons to push back protesters, led by opposition members of parliament, who had massed in front of a building where President Viktor Yanukovich was due to hold a press briefing.
They urged Yanukovich - who had planned a celebratory statement to crown the successful co-hosting of the Euro 2012 soccer tournament - to veto the bill, which was rammed through parliament late on Tuesday by the majority Party of Regions.
Yanukovich subsequently cancelled the briefing and instead called an urgent meeting with Parliament Speaker Volodymyr Lytvyn and leaders of major factions.
Lytvyn himself tendered his resignation at the opening of the parliamentary session.
The chamber approved the language bill in a second and final reading on Tuesday minutes after a proposal by one of the pro-Yanukovich deputies, giving opponents little time to cast their vote and prompting scuffles both in parliament and on the streets.
Many protesters stayed out on the streets of central Kiev overnight. When parliament met again on Wednesday, Lytvyn said: "Colleagues, I ask you to consider my resignation and take a decision on it."
The website of parliament said that Mykola Tomenko, a deputy speaker, had also stepped down. Tomenko belongs to jailed opposition leader Yulia Tymoshenko's BYuT parliamentary faction.
The bill, which will not become law until first Lytvyn and then Yanukovich have signed it, would upgrade the status of Russian in the former Soviet republic, where the official state language is Ukrainian.
People in large swathes of Ukraine, notably the eastern industrial heartland, speak Russian as their mother tongue.
But opposition parties and millions who speak Ukrainian as their first language see the bill as a potential threat to Ukrainian sovereignty and its 20 years of independence since the break-up of the Soviet Union.
If signed into law, the bill would recognise Russian as a "regional" language in predominantly Russian-speaking areas, enabling its use in the public service.
"This bill would push the Ukrainian language out of use," said one of the protesters, 40-year-old entrepreneur Yuri Chernyak. "It might be too late but we must do something and not stay indifferent."
Opponents of the bill say it was pushed through by Yanukovich's Party of the Regions in order to win back disenchanted voters in its Russian-speaking power base ahead of a parliamentary election in October.