KIEV, Aug. 5 – Tensions rose sharply in Ukraine on Friday after the arrest of former Prime Minister Yulia Tymoshenko, who is on trial for signing a controversial natural gas agreement with Russia in January 2009.

The arrest marks a significant escalation in pressure on Tymoshenko, Ukraine’s opposition leader, just as the criminal case against her had appeared to be close to collapse.

“The Rubicon has been crossed. Democracy is over. The government has turned into a regime,” Arseniy Yatseniuk, the leader of the opposition Front of Changes party, said in a statement.

“Today the regime has begun a war with its own people,” Yatseniuk said. “So far nobody in world history has won the war against its own people.”

The U.S. Embassy in Kiev has called on Ukraine to release the former prime minister. A statement issued Saturday also expressed concerns about the application of the rule of law in the country.

An official in Britain's Foreign Office, Alistair Burt, said Saturday his country will closely monitor Tymoshenko's trial and the cases against some of her former officials.

The EU's foreign policy chief Catherine Ashton and Enlargement Commissioner Stefan Fuele issued a statement late Friday calling for an independent and transparent trial.

The arrest came hours after Prime Minister Azarov testified at the trial, suggesting the gas agreement had caused significant losses for the Ukrainian economy.

In response, Tymoshenko accused Azarov of corruption and favoring RosUkrEnergo, a Swiss-based natural gas trader and the only supplier of gas to Ukraine for years until it was removed by Tymoshenko in January 2009.

After the agreement, RosUkrEnergo was replaced by Gazprom, which is now the only supplier of gas to Ukraine.

Tymoshenko’s arrest is viewed by her allies as a politically motivated pressure, and may trigger criticism from international human rights organizations and even foreign governments.

Tymoshenko’s party, Batkivshchyna, on Friday called on thousands of her supporters across Ukraine to flock to Kiev for a massive protest rally. The plan is for the protest rally to put heavy pressure on the authorities and, perhaps, to replace the government in September.

“We begin mobilization,” Oleksandr Turchynov, Tymoshenko’s closest ally, said Friday. “Today we begin an active battle.”

Lilia Frolova, the top prosecutor at the trial, requested the Kiev Pechesrskiy district court to arrest Tymoshenko for allegedly breaking rules of behavior at court sessions.

Tymoshenko, at the trial for the past 1.5 months, has openly ridiculed both prosecutors and Judge Rodion Kireyev, whom she had directly accused of implementing a political order from President Viktor Yanukovych.

“The court has repeatedly explained to the defendant her rights and duties at court sessions, but she continues to violate order in the courtroom, keeps insulting those present,” Frolova said. “By doing so she is delaying the process of establishing the truth.”

Kireyev, who agreed that Tymoshenko had been “disrespecting the court,” ruled on Friday she must be arrested. The next court session is scheduled on August 8.

This was the second request by prosecutors to arrest Tymoshenko. Kireyev denied the first request on July 27.

A speedy trial – and a guilty verdict – is believed to benefit Tymoshenko’s political opponents because she would not be able to run for a seat in Parliament at the next election in October 2012.

However, if the trial is delayed, at least through October, this would increase Tymoshenko’s political clout as many political figures may start to bet that her party would win many more seats in Parliament at the election.

A week ago, Tymoshenko said that Yanukovych has allegedly ordered the court to jail her before Ukraine celebrates the Independence Day on August 24.

Tymoshenko is accused of negotiating the gas agreement in January 2009 that had set a very high natural gas price for Ukraine that had threatened its economic recovery, according to Azarov.

Ukraine, led by Azarov and Energy Minister Yuriy Boyko, has been trying to change the agreement by negotiating lower gas prices, but Russia has refused to cooperate.

“Azarov did not give any answer to my questions, and prosecutors and court had begun to cancel my questions,” Tymoshenko told reported at the trial.

“I believe the silence of Azarov at the trial, as well as canceling of these questions, show that Azarov was a direct participant of these corruption schemes in the gas sector,” Tymoshenko said.

Tymoshenko accused Yanukovych of politically ordering her arrest as a way of punishing his political opponent.

“Of course, they can arrest me,” Tymoshenko said. “But the question that Yanukovych will be asked by the world and by Ukraine is ‘What for?” Tymoshenko said. (nr/sb/ez)