De Vriendt was racing in Wortegem-Petegem, and quickly received medical attention after collapsing, including 40 minutes of resuscitation efforts, but passed away. The cause of De Vriendt's death has been attributed to heart failure, though an autopsy has not been performed.
The race was the first one to take place in Belgium since the COVID-19 pandemic shut down the cycling season across Europe.
A social media post from a representative for the race confirmed the incident and its eventual outcome.
"Tragic news," read the post. "Niels de Vriendt, 20-year-old Belgian amateur cyclist of the VDM-Van Durm-Michiels-Trawobo CT team, passed away from heart attack in a simulate race at Wortegem - Petegem in Belgium.
"Resurrection efforts were made for 40 minutes, but Niels went into death. Due to this tragedy, the race was closed 13 kilometres from arrival. The Mayor of Wortegem - Petegem said there is no suspected cause, and an autopsy will only be performed if requested by the family."
De Vriendt's team posted a tribute message to Facebook on Sunday morning (AEST).
The death comes as part of a series of deaths from apparent heart problems within cycling, with Michael Goolaerts most recently suffering a heart attack during the 2018 Paris-Roubaix and dying.