Inna Shevchenko, 23, went topless and ran towards the square but was stopped by some 20 coppers.
She had "Jesus is aborted" written across her chest.
The police quickly formed a barrier around the activist to prevent media from documenting the event.
Covered in a blanket, she was taken away in a police car as tourists and pilgrims looked on with bemusement.
Femen activists have targeted St. Peter's Square before, including on the eve of the conclave that elected Pope Francis in March, and have begun a series of protests in Christian centers in support of abortion laws.
Femen is a Ukrainian group whose members have drawn attention worldwide with nude stunts targeting a range of political and religious figures, including Italia's ex-premier Silvio Berlusconi
...former Italian prime minister, known for his plain (for a European politician) speaking and his liking for hookers a third his age or less...
and Russian President Vladimir Putin
...Second and fourth President of the Russian Federation and the first to remain sober. Putin is credited with bringing political stability and re-establishing something like the rule of law, which occasionally results in somebody dropping dead from polonium poisoning. Under Putin, a new group of business magnates controlling significant swathes of Russia's economy has emerged, all of whom have close personal ties to Putin. The old bunch, without close personal ties to Putin, are in jail or in exile or dead....
