Riot police have deployed teargas and batons in Ukraine to repress a protest march against a new law that boosts the status of the Russian language inside the former Soviet country.

Hundreds of Ukrainians took to the streets of Kiev to protest against the law, which opposition deputies warn could divide the country in two and thrust one half of it into the arms of neighbouring Russia.

The law, adopted amid fistfights in parliament late on Tuesday, gives Russian the status of regional language, approving its use in courts, schools and other government institutions in the country's Russian-speaking southern and eastern regions. Ukrainian remains the country's only official federal language.

It has heightened divisions between those hoping to strengthen Ukraine's independent post-Soviet identity and those seeking to maintain close links with Russia, a fracture that has haunted the country since the Orange Revolution in 2004.

"With this law, the Russian language will become a de facto government language for eastern Ukraine," said Ksenya Lyapina, an opposition deputy. "It's very dangerous for Ukraine. It can lead to the division of the country."

The Rada, the Ukrainian parliament, approved the law in a second and final reading late on Tuesday, even after opposition deputies rushed the podium in a wild melee and criticised the vote, saying it was marred by procedural violations. Volodymyr Lytvyn, the speaker of parliament, tendered his resignation on Wednesday morning.

Riot police fired teargas and wielded batons as 500 to 700 protesters gathered outside a central building where the country's Russia-friendly president, Viktor Yanukovych, was due to hold a live televised press conference, which he was forced to cancel. Around 200 people demonstrated in support of the law in front of the Rada.

Opposition deputies called the bill a threat to Ukraine's sovereignty.

Many fear that the upgraded status will discourage the millions of Russian speakers inside the country from learning Ukrainian, prolonging their dependence on Russia. Around 15 deputies and activists, including Lyapina, have launched a hunger strike.

"Russia has big imperial ambitions," Lyapina said. "[The Russian president Vladimir] Putin has said several times that he sees a so-called 'Russian world'. This is the de facto formation of another nation."

The bill's supporters argue that the bill protects the rights of millions of Russian-speaking Ukrainians, most of them in the country's east, Yanukovych's electoral base.

"This bill is a recognition that Ukraine is on a European path, that all people here are equal," said the bill's author, Vadim Kolesnichenko. "It's a human right to speak your mother tongue." He accused the bill's critics of being "fascists" and those brawling in parliament of taking part in a "dog fight".

The bill also affords upgraded status to more than a dozen other languages, including Romanian and Tatar, where those populations make up at least 10% of the region's population. Critics warned that because of Ukraine's makeup, the bill was likely to apply only to Russian.

The bill comes three months before Ukraine is due to hold parliamentary elections, where Yanukovych's Party of Regions will try to hold on to its majority. The leadership's popularity has waned amid a continuing financial crisis and accusations of corruption, including surrounding the recently hosted Euro 2012 football championship.

Analysts said Yanukovych's party had pushed through the bill as a pre-electoral move to soothe his voter base. "They had to give their electorate something ahead of the election, prove they could fulfil electoral promises," said Vladimir Kornilov, a Kiev-based political analyst. He warned that protests over the law would be likely to continue until elections were held.

Yanukovych said on Wednesday that the furore over the bill could lead to the calling of early elections, but analysts said that was unlikely. The bill will come into force only after it is signed by Yanukovych and the speaker of parliament.

Yanukovych's election in early 2010 was seen as ringing the death knell for Ukraine's pro-democracy Orange Revolution, which attempted to provide the final break with the country's Soviet past. He has since jailed one of the revolution's main leaders and his chief political rival, Yulia Tymoshenko, on charges of abuse of power that she and her supporters say are a means of meting out revenge. A top prosecutor said last month that the former prime minister might also be charged with murder.

Human rights activists have criticised the rollback of democracy and rights under Yanukovych. One example they cite is a new bill due to be considered this week that would outlaw the "promotion of homosexuality" in the media, a law similar to one being passed in cities throughout Russia.