Demoitie's Wanty-Gobert team said the rider was hit by a motorbike following a fall during a section of the race in northern France on Sunday and later died in hospital.
He was appearing in his second world tour race, having made his first appearance in the E3 Harelbeke event in Belgium earlier in the week.
The team paid tribute to the 25-year-old after confirming his death, posting a black-and-white photograph with the text "Antoine Demoitie 1990-2016" on its official social media accounts.
The team said Demoitie was taken to the intensive care unit of a hospital in the northern French city of Lille and his wife and family had joined him there.
"Antoine joined Wanty-Groupe Gobert this season and wanted to ride WorldTour races," said team manager Jean-Francois Bourlart.
"Last Friday, he rode his first WorldTour race (the) E3 Harelbeke. He was part of the day's breakaway and climbed the Taaienberg with the best. He was very proud of himself and we couldn't have been more proud of him."
"On Sunday, the incredible happened, the inconceivable. We can't believe it."
Wanty-Gobert Sports director for Gent-Wevengem, Hilaire Van Der Schueren, explained the circumstances of the accident.
Antoine called me on the radio for bottles - his last words. I gave them and was called to a second group. I asked the commissaire to pass the group and then it happened.
"Our mechanic was the first to see Antoine and he immediately called me. At that moment I thought Antoine was dead but the ambulance was there straight away and he was airlifted to hospital.
"There was hope until the organization of Gent-Wevelgem asks me for contact details of Antoine's next of kin. Then you know it's very serious.
"After the race I immediately left for the hospital in Lille. I met Antoine's father who told me the doctors didn't have much hope. After midnight we received the news that Antoine had died. We must process this together."
The team has withdrawn from the Driedaagse De Panne-Koksijde race on Tuesday as well as the Route Adelie and Paris-Camembert but will race the Tour of Flanders on Sunday.
"This Wednesday we do a recon of Tour of Flanders together and then we eat together. We will talk about Antoine a lot," Van Der Schueren said.
CPA president Gianni Bugno, who won the world road race title in 1991 and 1992, said it was a time of "sadness and sorrow".
"The CPA and all the riders demand that light is shed on the accident immediately and the circumstances that caused it," the association said in a statement on Monday.
"We do not want to make controversy but we have so much frustration inside. We have always stated that the safety of the riders must come first.
"I do not want to accuse anyone but make everyone reflect on the responsibility we have to ensure that a very high level of attention and awareness is maintained plus control over safety standards during each race."
The sport's governing body, the UCI, said it would work with the relevant authorities looking into Demoitie's death.
"The UCI is extremely saddened to learn about the passing of Belgian rider Antoine Demoitie," it said. "Antoine will be truly missed," added UCI president Brian Cookson. "Our thoughts are with his family, friends and team."