Thousands of fans were expected to pay respects outside the the manor south of Paris where he is to be laid to rest near his beloved dogs.
The 88-year-old star of such classics as "Le Samourai" and "Purple Noon", who was once described as "Europe's James Dean", died on Sunday.
French police set up roadblocks near the manor in the village of Douchy, with the airspace overhead also closed for the entire of the weekend.
The 50 or so mourners allowed into the estate's private chapel will have to leave their mobile phones at the door.
But veteran Italian actress Claudia Cardinale, 86, who starred opposite Delon in "The Leopard", was "too sad" to come, her agent told AFP.
"They ask me to put into words (the grief)," she said after his death, "but the sadness is too intense".
Rosalie van Breemen, Delon's ex-wife and mother of his children Anouchka and Alain-Fabien, will be present, sources close to proceedings told AFP.
All week hundreds of fans have gathered outside the gates of La Brulerie to leave floral tributes and cards. Hours before the ceremony, a crowd had already begun to gather.
Delon's three children -- who were with him when he died -- told AFP that they were "extremely touched by the fervour and affection shown by his fans in France and across the world."
The ceremony is scheduled for 4:00 pm (1400 GMT).
Divisive figure
Since his death, France has been paying homage to Delon, one of the country's biggest but most divisive stars.
He was one of the last living legends of a golden era of French cinema in the 1960s.
While he had legions of fans around the world, Delon's relations with women caused controversy. His sons accused him of domestic violence, which Delon denied while admitting to slapping women.
The actor also drew criticism for supporting Jean-Marie Le Pen, co-founder of the far-right National Front, and his opposition to same-sex relationships.
Feminists were appalled by the lifetime achievement award the Cannes Film Festival gave him in 2019.
Delon lived his later years largely as a recluse, though his personal life kept him in the headlines.
In 2023, his three children filed a complaint against his live-in assistant Hiromi Rollin, accusing her of harassment and threatening behaviour.
The siblings went on to wage a public battle in the media and the courts, arguing over his health, which worsened after a stroke in 2019.
Even in death, he was still making headlines after it emerged that he asked for his favourite dog to be put down and buried with him.
But his longtime friend and fellow 1960s screen icon Brigitte Bardot said the Belgian malinois called Loubo would be spared.
"The family of Alain Delon have confirmed to us that they will take care of him. Loubo will of course not be euthanised," her foundation said on the X social media platform.
French TV presenter Stephane Bern said Delon's wish to be buried with his dogs was very him, comparing him to Frederick the Great of Prussia who did the same.
It was a gesture "of majesty and panache", he said, "very Delon, worthy of a Leopard who had become a misanthrope."
"I have absolutely no fear of death," the actor insisted in 2011, posing for photographs outside the tomb where he intended to be buried.
Despite his reputation as an inveterate womaniser both on and off the screen, Delon, a Catholic, said his "mad passion" was for the Virgin Mary.
"She is the woman I love most in the world," he said, "the one I talk most to."
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