Police in Ukraine's capital Kiev have fired tear gas to disperse hundreds of protesters angry over a new language law that boosts the status of Russian.
The clashes erupted outside a building where President Viktor Yanukovych was scheduled to give a speech.
The new law, drafted by Mr Yanukovych's Party of the Regions, was adopted by parliament on Tuesday without a debate on numerous amendments.
World-famous boxer Vitali Klitschko was among the activists hurt in the unrest.
The interior ministry said 10 anti-riot police from the elite Berkut unit were admitted to hospital with injuries. The ministry said protesters assaulted police with bottles and aerosol sprays.
The police were equipped with helmets, shields and batons.
Mr Yanukovych decided to postpone his speech on Wednesday as clashes continued.
Later he said he would have to call an early election if MPs failed to "stabilise parliament's work". A parliamentary election is officially scheduled for October.
Correspondents say about 1,000 opposition activists took part in the demonstration.
A BBC reporter saw people spattered with blood on both sides.
WBC heavyweight champion Klitschko called Tuesday's vote "political suicide" and urged opposition MPs to boycott parliament. He heads an opposition group called Udar (Blow).
The controversial vote prompted a request from Speaker Volodymyr Lytvyn to step down. "I have been fooled, Ukraine has been fooled," he complained.
Mr Lytvyn's deputy Mykola Tomenko also tendered his resignation.
President Yanukovych invited parliament leaders and heads of parliamentary factions to meet him to discuss the resignations.
Meanwhile, seven MPs angry at the vote have gone on hunger strike.
The bill will become law once signed off by President Yanukovych, who is seen by his critics as being close to Moscow. The bedrock of his support is in Russian-speaking eastern Ukraine.
The second reading vote took place despite scuffles in the chamber between the ruling party and opposition MPs.
The bill grants Russian, mother tongue of most people in east and south Ukraine, "regional language" status.
Critics fear it will dilute Ukraine's sovereignty and help return Ukraine to Moscow's sphere of influence.
While Ukrainian would remain the country's official language, Russian could be used in courts, hospitals, schools and other institutions in Russian-speaking regions. In practice Russian is already used widely in official establishments in Ukraine.
The new law says local officials can use a "regional language" if at least 10% of the local population are native speakers of that language.
Those officials would have to know the regional language and be able to use it in their official duties.
People will be allowed to choose which language they want their documents issued in - Ukrainian or regional.
The new law de facto grants Russian the status of an official language - but not the state one - in most of Ukraine.