ROME — A man managed to jump onto Pope Benedict’s uncovered popemobile yesterday in a dramatic breach of security that has triggered fears for the safety of President Bush when he visits the Vatican on Friday.

The 27-year-old German jumped over a protective barrier in St. Peter’s Square as Benedict was making his usual trip around the crowd before a general audience.

The pontiff appeared not to notice the man as he clung onto the back of the popemobile for several seconds before being wrestled to the ground by eight security officers. Benedict did not look back to see the cause of the commotion, and he proceeded with the audience as planned.

The unnamed man was not trying to attack the pope, the Vatican said, but he showed “clear signs of mental instability.”

Father Federico Lombardi, the papal spokesman, said, “The man wanted to get near to the pope’s vehicle, not because he was making an attempt on the pope’s life, but in order to attract his attention.”

After his arrest, the man was taken for questioning by a Vatican judge, Gianluigi Marrone, who concluded that he should be treated for his mental condition. “Psychiatrists from the Vatican have taken him for special treatment in a specially protected building nearby. The case is closed,” Father Lombardi said.

Security in the Vatican has been tightened since the September 11, 2001, attacks, and police vet all visitors entering St. Peter’s Square when the pope holds an audience.

However, audiences are open to the general public, and when the pope is in the Vatican, his popemobile is uncovered. When he travels outside the Holy See, it is usually fitted with bulletproof glass.

St. Peter’s Square is cordoned off with barriers to create a route among the faithful, who numbered around 35,000 yesterday.