MANILA, Philippines (AP) — 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.