By Lucien Libert

PARIS (Reuters) - An 78-year-old French far-right activist committed suicide at the altar of the Notre Dame cathedral on Tuesday by shooting himself in the mouth, three days after a law legalizing same-sex marriage came into effect.

Police evacuated the cathedral, one of Paris' biggest tourist draws, after Dominique Venner - a historian known for his hard-right political essays and a fierce opponent of gay marriage - shot himself, sending tourists fleeing in panic.

Venner made no declaration as he shot himself around mid-afternoon, a police source said. He carried a letter on his person, but its contents were not released to the media.

A May 21 entry on his blog page appealed to readers to join a march planned for Sunday against the Socialist government's gay-marriage law, which came into force at the weekend.

President Francois Hollande made good on a campaign pledge by making France the 14th country to allow same-sex weddings. The country's biggest social reform in three decades is backed by a small majority of French, surveys show.

Yet the move stirred up a storm of opposition among social conservatives and Catholics who have hosted a series of angry and often violent protests.

Demonstrators decked in baby pink and blue have marched repeatedly through Paris, leaving slogans stamped on pavements reading "We want jobs, not gay marriage".

Venner fought for France in the 1954-62 Algerian War of Independence. He later became a hard-right activist and author specializing in military and political history.

Far-right National Front leader Marine Le Pen, who scored nearly a fifth of the first-round vote in the 2012 presidential election, paid tribute to him on Twitter, saying his suicide was a political gesture aimed at "waking up the French people".

A few dozen readers tweeted a link to his last blog post.

Interior Minister Manuel Valls arrived at Notre Dame soon after police and emergency services and rued the fact Venner had chosen the emblematic cathedral as a venue for his suicide.

"Notre Dame is Paris' cathedral, it's one of the most beautiful symbols of our capital and our country," he said. "We are fully aware of the repercussions of such an act."

Dominating a tiny island in the Seine river, in the heart of Paris, the 850-year-old cathedral was packed with hundreds of visitors who jostled to get out after the shooting, according to local student Prince Salabanzi, who was there to attend mass.

"All of a sudden he shot himself in the head -- everyone was traumatized," he told Reuters.

The incident came days after a man killed himself with a sawn-off shot-gun at a primary school in an upmarket Parisian neighborhood in front of children and a teacher. The motive for that suicide was unclear.

(Additional reporting by Lucien Libert and Nicolas Vinocur; Writing by Leigh Thomas; Editing by Catherine Bremer)