The trial of Gen. Fabian Ver and 25 other men linked to the assassination of Philippine opposition leader Benigno Aquino opened today with pathologists and forensic experts leading a list of witnesses.

Security was tight, and journalists and spectators were searched thoroughly before they could enter the courtroom for the trial. Sheriff Edgardo Urieta said the three judges hearing the case had received several bomb threats.

Security men filled the courtroom, and plainclothesmen sat in every row of seats reserved for the public. The accused, surrounded by uniformed escorts, and security men snapped to a salute when Ver, 65, entered the courtroom.

Riot police, some armed with machine guns, stood guard outside, but there were no incidents. Fifteen demonstrators, carrying placards outside the courthouse,said the accused were receiving better treatment than student leaders arrested recently in demonstrations.

Ver, a close confidant of President Ferdinand Marcos, is charged with covering up the assassination, as are Manila's police chief, Maj. Gen. Prospero Olivas, and six other soldiers. The charge carries a maximum penalty of 20 years in jail.

Seventeen other military men, including Brig. Gen. Luther Custodio, head of the security unit at Manila Airport, where Aquino was killed Aug. 21, 1983, face possible death sentences on charges of direct involvement in Aquino's death and that of Rolando Galman. The military blamed Aquino's assassination on Galman, who they said was a professional gunman hired by communist rebels. Galman was shot immediately after Aquino was slain.

One businessman charged as an accomplice faces a maximum penalty of life imprisonment.

All 26 men have pleaded not guilty. They are either out on bail or are confined to military quarters.

In October an official fact-finding panel, after spending almost a year investigating the slayings, implicated the 26 men in the murder of Marcos' political archrival. The majority of the panel concluded, based largely on circumstantial evidence, that Aquino was killed by one of his mililtary escorts on a stairway leading from the airplane, and that Galman was set up to take the blame.

In testimony today, Pedro Solis, former technical services director of the National Bureau of Investigation, a police agency, said lead fragments recovered from Aquino's chin "were in all likelihood parts of a .38 or .45-caliber bullet . . . . "

The military had said a .357 magnum revolver was used to kill Aquino, who returned to Manila after three years of voluntary exile in the United States.

A government pathologist, Dr. Bienvenido Munoz, who performed the autopsy on Aquino, repeated his finding that the opposition leader died from one bullet fired from behind his head from the muzzle of a gun pointed upward. Munoz said his investigation did not show whether Aquino was shot from the ground or a stairway.

The cross-examination of the witnesses will be limited to the evidence they gave the panel.