* Organisers say 200,000 protestors in Paris; police say 70,000
* Pope Benedict tells French bishops church's voice must be heard
By Gérard Bon
PARIS, Nov 17 (Reuters) - Tens of thousands of people, joined by Catholic church leaders, marched in cities across France on Saturday to protest against government plans to legalise same-sex marriage.
Demonstrators, holding banners with slogans such as "don't touch civil marriage", "all born from the union of a man and a woman" and "one father + one mother for all children", took to the streets in Paris, Lyon, Marseille and other major towns.
The organisation behind the protests, called "Demos for All" in imitation of the "marriage for all" call of gay-marriage campaigners, says the Socialist government's reforms, which polls indicate have popular support, would threaten "major and dangerous upheaval".
The organisers said 200,000 people demonstrated in the French capital, compared with a police estimate of 70,000.
France's National Assembly is due to start examining the proposed law allowing same-sex marriage in January, with a vote expected in mid-2013. It would grant gay couples the right to adopt children but not to use assisted procreation methods.
Recent polls show the majority of French people support gay marriage, though not the right for homosexual couples to adopt.
France's top Catholic prelate, Paris Cardinal Andre Vingt-Trois, earlier this month criticised the government for forging ahead with the plans at a time when the country faced urgent economic concerns.
Pope Benedict told French bishops visiting the Vatican on Saturday not to be "afraid" of spreading Christian teachings.
"In the important debates about society, the voice of the church must make itself heard relentlessly and with determination," he said.
The Vatican pledged this month never to stop fighting attempts to "erase" the privileged role of heterosexual marriage, which it called "an achievement of civilisation".
Jean-Francois Cope, who is hoping to become leader of the opposition centre-right UMP party in an internal contest with ex-Prime Minister Francois Fillon on Sunday, sent a letter of support to the protesters.
While he said he would not join them, other elected UMP representatives and mayors were among demonstrators in Paris.
Lyon Cardinal Philippe Barbarin, who has said marriage was defined at the very start of the Bible as created by God to join man and woman, was one of around 10,000 protesters in that city, according to figures from the organisers.
"I'm here like everyone else, like the Protestants, the Catholics, the Muslims, the philosophers, it's a message to politicians so that a debate is organised on such a fundamental issue," he said, adding that he had come as a "simple citizen".
The archbishop of Toulouse, Robert Le Gall, joined 10,000 there, according to organisers. Police said there were 5,000.