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Bullit Marquez, Pool, Associated Press
Congressman Niel Tupas stresses a point as he argues for the prosecution at the start of the impeachment trial for Philippine Supreme Court Chief Justice Renato Corona in Manila, Philippines, Monday, Jan. 16, 2012. The impeachment trial of Corona began Monday after he was accused of corruption and trying to block the prosecution of former President Gloria Macapagal Arroyo.

MANILA, Philippines — The Philippines' first impeachment trial of a Supreme Court justice opened Monday, in a major battle of President Benigno Aquino III's anti-corruption campaign targeting his detained predecessor and her allies.

Chief Justice Renato Corona was impeached by the House of Representatives last month on corruption allegations as well as accusations that he tried to block prosecution of former President Gloria Macapagal Arroyo, who has been detained on vote-rigging charges.

Corona's trial before the Senate has sparked fears of a constitutional crisis pitting Aquino against the 15-member court, where 12 of the justices were appointed by his rival and predecessor, Arroyo.

Senate President Juan Ponce Enrile promised his chamber would hold an impartial trial as the nationally televised proceedings got under way Monday, attended by Corona, his wife and a dozen defense lawyers. In his first ruling, Enrile dismissed a Corona petition that may have halted the trial on grounds the charges against him were defective.

More than 300 left-wing activists demanding Corona's conviction rallied outside the Senate, a block away from dozens of Corona followers. Police anti-riot squads stood by to prevent any clashes.

Rep. Niel Tupas Jr., who leads a congressional team of prosecutors against Corona, said the chief justice was impeached due to eight acts of alleged corruption and improperly issuing decisions that favored Arroyo.

"We are not here to indict the Supreme Court as an institution," Tupas told the senators. "We are here because one man — Chief Justice Renato Corona — has bartered away for the pot of porridge the effectiveness, independence and honor of the Supreme Court."

A defiant Corona led a rally by hundreds of employees and judges at the Supreme Court before the trial, denying any wrongdoing and vowing to defend the high tribunal's independence.

"I have not sinned against the president. I have not sinned against the people. I have not stolen from anyone," Corona told his cheering followers. He expressed confidence he would be acquitted.

Aquino, son of revered pro-democracy icons, has accused Corona of trying to derail Arroyo's prosecution and his reforms, which are aimed at battling pervasive corruption and poverty.

Aquino said he expected Corona to be convicted due to the strength of the evidence against him.

The trial is the latest struggle in a political drama pitting Aquino against Arroyo, whom he blames for a decade of corruption scandals that eroded public trust in government and held back foreign investors.

Corona was Arroyo's chief of staff before she appointed him chief justice shortly before her stormy, nine-year rule ended in 2010. Corona has accused Aquino of acting like a dictator by maneuvering to oust him to gain a rubber-stamp Supreme Court.

A Philippine regional trial court ordered the 64-year-old Arroyo arrested on Nov. 18 after she was charged with ordering the rigging of 2007 senatorial elections to favor her candidates. She has denied any wrongdoing and is detained in a government hospital while awaiting trial.

The country's largest group of lawyers has backed Corona, accusing Aquino's allies of improperly rushing his impeachment. Aquino's administration was endangering democracy by undermining the independence of the judiciary, the Integrated Bar of the Philippines said.

Presidential spokesman Edwin Lacierda said the trial will shed light on alleged wrongdoings by a chief justice, a subject largely regarded as taboo. "We believe that the truth will set the judiciary free," he said.

Associated Press writer Teresa Cerojano contributed to this report.