Acting on a judge's orders, police arrested former Ukraine Prime Minister Yulia Tymoshenko during her abuse-of-office trial on Friday for violations of court procedures.
Her supporters in court, including national lawmakers, squabbled with riot police, trying to prevent them from driving her away in a prison car and shouting: "Shame! Shame!"
Dozens of Tymoshenko's supporters then gathered outside the court building in central Kiev and tried to block the road, but riot police pushed them aside.
The United States and the European Union have condemned court cases against Tymoshenko and several of her top allies as selective prosecution of political opponents. Germany on Friday expressed "great concern" about Tymoshenko's arrest.
Viktor Yanukovych, Ukraine's Kremlin-friendly president, has been accused by opposition activists and rights groups of trying to sideline his political opponents and muzzle critical media in a rollback on freedoms championed by his pro-Western predecessor.
The charismatic Tymoshenko, the country's top opposition leader, has criticized the trial as an attempt by Yanukovych to bar her from elections and mocked the court. Tymoshenko has refused to rise when addressing the court, as required, and routinely insulted the judge. Her supporters have repeatedly disrupted hearings.
Complying with the presiding judge's orders, police surrounded Tymoshenko and escorted her out of the courtroom on Friday. Hundreds of police officers surrounded the prison car as it forced its way slowly through the crowd of Tymoshenko supporters.
Later, Tymoshenko's ally, Serhiy Vlasenko, showed reporters what he called a handwritten note from her describing her arrest as a "political repression."
The 50-year-old opposition leader is charged with abusing her powers by signing a natural gas import contract with Russia in 2009 that prosecutors claim was disadvantageous to Ukraine.
Tymoshenko insists she is innocent, arguing that the contract ended weeks of natural gas disruptions to Ukrainian and European consumers and that she was authorized to sign the deal as prime minister.
On Friday, Tymoshenko was put in the same jail where one of her allies, former Interior Minister Yuri Lutsenko, has been held for more than six months on charges that he defrauded the government when he paid his driver illegal bonuses.
Experts in Ukraine and abroad believe the trial's real motive for Tymoshenko's trial is to disqualify Tymoshenko from upcoming parliamentary and presidential elections by convicting her as a felon.
Tymoshenko was nicknamed Ukraine's "gas princess" in the late 1990s for serving as the head of a leading natural gas importer. She was jailed briefly in 2001 on suspicion of smuggling gas, but the case never went to trial.
Tymoshenko has a long and bitter history with Yanukovych.
She was the central figure in Ukraine's 2004 Orange Revolution mass protests that threw out Yanukovych's fraud-tainted victory in a presidential election and led to another vote that brought a pro-Western government to power.
Tymoshenko became prime minister, but Ukrainians grew frustrated by economic hardships, slow reforms and endless bickering in the Orange camp. As a result, she lost to Yanukovych in the 2010 presidential election.
Tymoshenko's party said it would mobilize its supporters and call street protests.
One ally, Serhiy Pashinsky, said: "Tymoshenko's arrest is a verdict against the government and the beginning of its end."
Russia's Foreign Ministry said in a statement Friday that the 2009 gas deal was signed in strict conformity with Russian and Ukrainian law on orders from the two nations' presidents. In what sounded like a note of criticism, the ministry said Russia expects Tymoshenko's trial to be fair and in line with "elementary humanitarian norms."
While Yanukovych is generally seen as Moscow-friendly, his government has continued to argue with Russia over gas prices and other economic issues.
German Deputy Foreign Minister Werner Hoyer expressed "great concern" at Tymoshenko's arrest and said there were "doubts as to whether the investigative custody is proportionate."
"The fact that numerous members of the previous government are being investigated for abuse of office awakens a suspicion of politically motivated justice," Hoyer said in a statement. "If this impression is strengthened further, that would be a significant hurdle to Ukraine moving closer to the European Union."
Geir Moulson in Berlin contributed to this report.