FOUR activists from Ukrainian feminist group Femen have stripped off in St Peter's Square in a protest for gay rights just as Pope Benedict XVI was reciting his traditional weekly Angelus prayer.

The four topless women had "Shut Up!" scrawled on their fronts and "In Gay We Trust" on their backs.

"Homophobe shut up!" the activists shouted as the Pope was speaking on Sunday.

Several Italian and Vatican police officers quickly descended on the scene and detained the activists.

They staged their brief protest by the giant Christmas tree in the centre of the square to the dismay and curiosity of a crowd of pilgrims who had come out to see the Pope on a rainy winter day.

Tens of thousands were set to march in France later on Sunday to denounce government plans to legalise gay marriage and adoption, which have angered many Catholics and Muslims, France's two main faiths.

A small protest against the plans was also due to be held in Rome outside the French embassy.

The Femen women's power group has been making headlines since 2010 for topless feminist, pro-democracy and anti-corruption protests in Russia, Ukraine and Britain.

In September, they set up their first ever "training centre" in Paris.