PARIS (AFP) - Fifty opponents of gay marriage were arrested in central Paris late Saturday, police said, on the eve of a major protest against a new French law allowing homosexual couples to formally tie the knot.

The demonstration on Saturday took place along the French capital's iconic Champs-Elysees, a week after France became the 14th country to legalise same-sex marriage.

Police expect 200,000 people to attend the rally on Sunday against gay marriage, with the crowds expected to include hundreds of "ultras", who authorities believe could cause trouble.

President Francois Hollande signed the gay marriage bill into law on May 18 following months of bitter debate and demonstrations. France's Constitutional Council threw out a legal challenge by the right-wing opposition to the new bill, which also legalises gay adoption.

But while rights groups have hailed the move, opponents of the measures have vowed to fight on and on Saturday evening a group of protesters chained themselves to metal barriers they placed in the middle of the busy Champs-Elysees, Some fired smoke cannisters before police moved in and arrested them.

At the same time dozens more protesters assembled on the pavement waving flags.

In total 50 people were held and a van carrying masks, banners and smoke bombs was seized, police said.

The incident passed off without violence, police sources added.