A 78-year-old man committed suicide on Tuesday in front of the altar of Notre Dame Cathedral in Paris, police say, causing its evacuation.
Dominique Venner published a blog post on the same day of his suicide about "new gestures, spectacular and symbolic ones, to rattle lethargies, to shake anaesthetized consciences and awaken the memory of our origins."
He has recently been campaigning against gay marriage in France, is an award-winning far-right historian.
On Saturday, President Francois Hollande signed the bill into law.
Police said Venner had made no statement before killing himself, although a note was found next to his body. They did not disclose its contents.
Earlier on Tuesday, he had written on his blog a damning critique of the same-sex marriage bill.
"New spectacular and symbolic actions are needed to wake up the sleep walkers and shake the anaesthetised consciousness," he wrote.
"We are entering a time when acts must follow words."
Venner is also a former member of the Secret Army Organisation (OAS), which opposed Algerian independence in the early 1960s and tried to assassinate Charles De Gaulle, according to Telegraph.
The BBC's Christian Fraser in Paris says Notre-Dame is the most visited landmark in France, attracting more than 13 million visitors each year, but security is relatively relaxed.
It would not be difficult to conceal a weapon in a shoulder bag, he says.
The cathedral is celebrating its 850th year, and at the time of Mr Venner's death, it would have been busy, according to BBC's correspondent.
Police said the evacuation began immediately, that there were no further problems, and that the cathedral for the moment remained closed.
"It's unfortunate, it's dramatic, it's shocking," the rector of Notre-Dame, Monsignor Patrick Jacquin, told the Associated Press news agency.
This was the first suicide in decades at the cathedral, he said. A few people had jumped to their deaths from Notre-Dame's twin towers, but no-one was thought to have killed themselves at the altar before, he added.
"We will pray for this man, as for so many others at their end."
Last Thursday, a 50-year-old man with a history of mental problems killed himself with a sawn-off shotgun in front of a dozen children at a private Catholic school next to the Eiffel Tower.