* Muslim activists, UOIF group back Catholic-led protest
* Religious leaders leave major protest to lay groups
* Opponents plan three marches meeting at Eiffel Tower
French Muslims have begun joining a mostly Catholic-led movement against same-sex marriage, widening opposition to the reform that the Socialist-led government is set to write into the law by June.
Fifty Muslim activists issued an open letter on Monday urging fellow Muslims to join a major Paris protest against the law on Sunday. That followed a similar appeal last Saturday by the influential Union of French Islamic Organisations (UOIF).
Leaders of almost all main faiths in France have spoken out against the law, but not called on their followers to march in Sunday's demonstration to avoid giving the opposition campaign an overly religious tone.
President Francois Hollande and his government clashed with the Catholic Church last weekend, telling Catholic schools not to discuss the law with their pupils and urging state education officials to report anti-gay discussions at Catholic schools.
"We will protest on Jan. 13 by joining a pluralist campaign to preserve the traditional framework of marriage," the Muslim activists' letter said. "We invite all French Muslims to turn out in large numbers."
The UOIF statement also urged Muslims to join the "March for All", the Paris protest against the reform the government has dubbed "Marriage for All".
"This bill, if it passes, will disrupt family and social structures and civil law dangerously and irreparably," it said.
The Muslim activist letter was signed by intellectuals, business leaders and leaders of several grassroots Muslim groups. It accused the government of using the marriage issue "to mask its ineffectiveness in the fight against unemployment".
France's 5-million-strong Muslim minority is Europe's biggest and Islam is the second largest faith after Catholicism.
FAITH LEADERS STAY AWAY
The government has a comfortable majority in parliament to pass the bill. Opinion polls show almost 60 percent of the French support same-sex marriage but less that half want to let gay couples adopt children, which is part of the reform.
Catholic bishops began demanding a public debate on the reform last summer and were soon joined by leaders of the Jewish, Muslim, Protestant and Orthodox Christian communities.
In response, the government organised committee hearings in parliament but kept a low public profile in the debate until a protest march in November rallied 100,000 against the law.
While the Catholic hierarchy has not officially backed next Sunday's protest, eight bishops plan to join the march, 25 have encouraged Catholics to attend it and another 40 have defended it, the Catholic daily La Croix reported.
Cardinal Andre Vingt-Trois, head of the French Church, Chief Rabbi Gilles Bernheim and French Muslim Council head Mohammed Moussaoui have all decided not to join the street protest.
"When I need to make something known to the government or the president, I don't need to go demonstrate," Vingt-Trois said last month.
The French Buddhist Union is the only main faith group that declined to take a stand, saying Buddha never spoke about homosexuality and suggesting a referendum to decide the issue.
The march, which supporters say could draw up to half a million protesters, will start at three different points around Paris and meet at the Eiffel Tower.