Tens of thousands of people gathered in Rome's Circus Maximus arena to protest against a civil unions bill for same-sex couples, a hot-potato issue for Prime Minister Matteo Renzi's government.

But while organisers had been hoping to attract one million people and authorities had prepared for 500,000, journalists at the scene estimated the numbers to be in the tens of thousands. Official numbers were not immediately available.

"Without limits, our society will go mad!" organiser Massimo Gandolfini told the "Family Day" rally, as grandparents, parents and children held up banners reading "Wrong is wrong" in the capital's ancient Roman chariot racing stadium.