VATICAN CITY, OCT. 30 -- An American climbed over a barricade in St. Peter's Basilica during a mass today and ran toward Pope John Paul II shouting antinuclear slogans in Italian before he was restrained by papal security agents.

The man involved in the brief incident was identified as Thomas Siemer of Columbus, Ohio, who has gained notoriety with unconventional public appeals to the pope for action on nuclear disarmament.

His activities have included the authorship of a counterfeit papal letter on peace.

Siemer, a former contracts administrator with the missile division of Rockwell International, was restrained by Vatican security agents before he could reach the altar. Witnesses said Siemer was unarmed.

He was detained, but later released.

Officials said Siemer disrupted the mass, which marked the conclusion of a month-long synod of bishops on the Catholic laity.

Siemer shouted "something like, 'Holy Father, please work to ban nuclear weapons'," said Joseph Scottino, of Erie, Pa., executive secretary of the Fellowship of Catholic Scholars.

Siemer worked in the nuclear arms industry until 1976, when he joined the disarmament movement.

In August 1983, he and 13 other pacifists staged a 40-day hunger strike, urging the pope to condemn the production and testing of nuclear arms.

Siemer recently sent a statement to the Rome office of United Press International in which he admitted his authorship of a counterfeit encyclical on peace.

The fraudulent papal letter, titled "The Resurrection of Peace," declared that it was a sin for Catholics to help build or deploy nuclear arms.

In Columbus, a spokesman for the Catholic diocese said Siemer was affliated with an Catholic-based peace group called Pax Christi.

Jack Kirby, chairman of the Columbus-based Commission on Justice and Peace, said, "In some ways he's been sort of a gadfly in terms of the church itself and those causes he feels very close to."