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 the 25-year-old died in hospital.
"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.
CPA president Gianni Bugno, who won the world road race title in 1991 and 1992, said it was a time of "sadness and sorrow".
"We do not want to make controversy but we have so much frustration inside," added the Italian. "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," Bugno said.
Demoitie 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 sport's governing body, the UCI, said it would work with the relevant authorities looking into his 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."
The 243km (151-mile) Gent-Wevelgem, which starts and ends in Belgium, was won by world road race champion Peter Sagan of Slovakia.
(Reporting by Philip Blenkinsop and Tony Jimenez; Editing by Louise Heavens and Ken Ferris)
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