Ukraine: Demonstrators rally against new law curbing protests NEW: An opposition leader says the President called him about negotiations 30 police officers were injured in clashes, the Interior Ministry says Interior Ministry says some protesters tried to march on government buildings The new law was passed by lawmakers loyal to Ukraine's president in a show of hands Kiev, Ukraine (CNN) -- Thousands joined a rally in Kiev's Independence Square in freezing temperatures on Sunday, in defiance of new laws that set limitations on the right to protest. Ukraine's Interior Ministry said that after the rally, some protesters sought to take over government buildings and clashed with police, who blocked the street. The protesters, wearing masks and helmets, used flares, tear gas, baseball bats and stones in a confrontation with police, breaking doors and windows on police buses, the Interior Ministry statement said. Estimates of the number of protesters in the capital's square varied widely. Both protesters and police accused each other of attacks. The ministry said 30 police were injured, and one was taken hostage by the protesters. Four protesters were detained by police, the Interior Ministry said. A post on a Facebook page for Olesia Orobets, an opposition lawmaker, said she had been attacked by police and her phone had been taken away. Sunday's protest marked the latest fallout over legislation that was approved Thursday by lawmakers loyal to Ukraine's pro-Russian President Viktor Yanukovych, who signed it into law soon afterward. Amid the clashes, one opposition leader said he had received word that the government was prepared to negotiate. Opposition leader Arseniy Yatsenyuk said Sunday that he had received a call from Yanukovych saying the government was prepared to negotiate with the opposition. Ukrainian government officials did not immediately comment. The clashes and possible move toward talks come in the wake of weeks of public protests after Yanukovych's decision in November to spurn a planned a trade deal with the European Union and turn toward Russia instead. The new law includes provisions barring people from wearing helmets and masks to rallies and from setting up tents or sound equipment without prior police permission. Opposition politicians have objected to the way that Yanukovych's Party of Regions pushed the bill through the parliament, the Verkhovna Rada. Vitali Klitschko, an opposition leader and former heavyweight boxing champion, said the law -- which was adopted by a show of hands -- was "invalid," according to his UDAR party's website. He stood between the police and the crowd in the square, trying to prevent violence. The far-right Svoboda Party said in a news release that people were "outraged by the Party of Regions actions at the Verkhovna Rada and their recently adopted draconian laws. Protesters are demanding resignation of the government and asking the authorities to stop the persecution of activists. Kerry: 'Anti-democratic' steps U.S. Secretary of State John Kerry also criticized the new legislation in remarks Friday, saying it was "rammed through" the parliament "without transparency and accountability" and that it "violates all the norms" of the European Union. "We believe deeply that the people of Ukraine want to affiliate and want to be associated with Europe and they want to turn in that direction," Kerry said. And the steps that were taken yesterday are anti-democratic, they're wrong, they are taking from the people of Ukraine their choice and their opportunity for the future. In December, despite weeks of protest by anti-government protesters, Yanukovych agreed to a deal with Russia's President Vladimir Putin for Moscow to buy Ukrainian debt and slash the price Kiev pays for its gas. The tumult in Ukraine goes to the heart of its future ties with Russia and the rest of Europe. Ukraine is split between pro-European regions in the west and a more Russia-oriented east. The protests have unfolded since November 21, when Yanukovych changed his stance on the EU trade pact, which had been years in the making. The demonstrators say an EU agreement would open borders to trade and set the stage for modernization and inclusion. Ukraine's government said the terms needed to be renegotiated to better protect Ukrainians. Journalist Victoria Butenko reported from Kiev and Laura Smith-Spark wrote in London. CNN's Joseph Netto contributed to this report.