French police says a 78-year-old man, named as Dominique Venner, has killed himself inside the cathedral of Notre-Dame de Paris, causing its evacuation.
The man pulled out a shot-gun and shot himself through the mouth beside the main altar shortly after 16:00.
He has been named as Dominique Venner, an award-winning far-right historian.
Dominique Venner had recently been involved in the campaign against the government’s decision to legalize same s** marriage.
On Saturday, President Francois Hollande signed the bill into law.
Police said Dominique 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 anaesthetized consciousness,” he wrote.
“We are entering a time when acts must follow words.”
Dominique Venner is also a former member of the Secret Army Organization (OAS), which opposed Algerian independence in the early 1960s and tried to assassinate Charles De Gaulle.
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.
The cathedral is celebrating its 850th year, and at the time of Dominique Venner’s death, it would have been busy.
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.