The Pope was not hurt in any way and continued to hold the audience as if nothing happened, reports CBS News correspondent Sabina Castelfranco.
At least eight security officers who were trailing the vehicle as it moved slowly through the square grabbed the man and wrestled him to the ground. The pope didn't even look back.
The man "looked a little crazy," said the Rev. Federico Lombardi, a Vatican spokesman. He said the man was being held for questioning by Vatican police.
The man, a 27-year-old German whose name was not released, was wearing a pink T-shirt and dark shorts, a beige baseball cap and sunglasses. He appeared to have vaulted himself up and over the barricade from the second or third row back. He got as far as the back of the jeep, holding onto it for a few seconds, before being wrestled to the ground.
The jeep kept moving, and Benedict kept waving, then proceeded with the audience as if nothing had happened.
Since the Sept. 11, 2001, attacks, the Vatican has tightened security in St. Peter's Square when the pope is present. All visitors must pass by police to get into the square, with some walking through metal detectors or being searched with metal- detecting wands.
Nevertheless, virtually anyone can attend the audience. While tickets are required, they can often be obtained at the last minute — particularly in good weather when the audience is held outside in the piazza.
When the pope uses the popemobile in St. Peter's, it is usually uncovered; when he travels overseas or outside the Vatican, he usually uses one outfitted with bulletproof glass.
The pope is protected by a combination of Swiss Guards, Vatican police and Italian police.
On Wednesday, the head of the Swiss Guards, Col. Elmar Maeder, walked along one side of the popemobile while the pontiff's personal bodyguard, Domenico Giani, took the other side. Several plainclothes security officials trailed them.
Benedict stood up behind the driver, holding onto a bar to steady himself, with his personal secretary, Monsignor Georg Ganswein, seated behind him.
St. Peter's is cordoned off with wooden barriers to create "routes" that the popemobile can drive along to make the pontiff more visible to the crowd, which on Wednesday numbered about 35,000.
From his perch on the jeep, the pope waves and blesses the crowd, and occasionally will bless a baby handed up to him by a security guard. The jeep, though, never stops, with security officials walking or jogging alongside the whole way.
Benedict's predecessor, in the abdomen on May 13, 1981, in St. Peter's Square by Mehmet Ali Agca of Turkey. Agca was caught and served his sentence in Italy before being transferred to Turkey. He was released from prison in early 2006.