Wearing tight black jeans and a see-through black top, Oleksandra Shevchenko managed to slip into St. Peter's Square in the heart of the Vatican.
She held up a placard demanding "Freedom For Women," struck provocative poses, chanted "Freedom! Freedom! We Are Free!" in Italian, and removed her shirt before being surrounded and detained by police.
Two other Femen members were detained before they could reach the square, Interfax reported.
Shevchenko's protest took place just moments after the pontiff had delivered his traditional Sunday blessing to pilgrims in the square.
WATCH: A protester from the Ukrainian women's rights group Femen is arrested in the Vatican following a provocative protest against "papal patriarchal propaganda."
A statement on the group's website said the stunt was a "protest against papal patriarchal propaganda," which "imposes medieval ideas about women on the world."
Femen has long scandalized Ukraine with its topless protests, most recently with a demonstration against the jailing of former Prime Minister Yulia Tymoshenko.
Recently they have taken their show on the road, with a series of protests in Europe. In Paris, they protested in front of the home of former IMF Director Dominique Strauss-Kahn and in Rome they painted their topless bodies in the color of the Italian flag while calling for the resignation of Prime Minister Silvio Berlusconi.
-- Brian Whitmore