A woman identified by her lawyer as the niece of new Philippine President Corazon Aquino pleaded guilty yesterday to possession of an unregistered firearm after a pistol was confiscated from her briefcase at the Hart Senate Office Building.

Esther Chen entered the plea before D.C. Superior Court Judge Bruce Beaudin after her lawyer explained that the 36-year-old Connecticut resident felt she needed the gun because she feared political retaliation. The lawyer said Chen did not know she was violating the law.

Chen and other Aquino supporters had traveled to Washington on Monday to lobby congressional leaders to support Aquino's attempt to oust Philippine President Ferdinand Marcos following last week's disputed election results.

Chen was arrested Monday afternoon after another Aquino supporter tried to enter the Senate building carrying Chen's briefcase. The man carrying the briefcase said Chen was the owner after the .25-caliber automatic pistol was detected by an X-ray machine. The man was not charged.

Lawyer Cary Feldman told Judge Beaudin that a combination of factors had prompted Chen to bring the pistol to Washington, including the kidnaping and murder of a cousin last week in the Philippines and threats made against Chen.

The charge carries a maximum penalty of a year in jail and a $1,000 fine. Beaudin said he would not sentence her now, but would reserve the right to do so later.