Coquard looked to have the sprint wrapped up, but Debusschere snatched victory at Dwars door Vlaanderen by a bike throw.
Jens Debusschere wins his first classic on a dark day in Belgium, out-sprinting Bryan Coquard, who sat up too soon to celebrate.
The cobbled semi-classic is held roughly 100km west of Brussels, the site of Tuesday's horrific terror attacks.
Race officials say that Dwars door Vlaanderen will be held on Wednesday, pending final approval from Belgian authorities.
Explosions at the Brussels airport and a metro station killed more than two dozen people, and now this week's classics are in jeopardy.
After a training camp on the cobbles and successful sorties at Dwars door Vlaanderen and E3 Harelbeke, Quintana feels prepared for the Tour
Topsport-Vlaanderen Baloise plays team tactics to perfection to go one-two in Wednesday's Belgian semi-classic
Topsport Vlaanderen-Baloise take first and second at Dwars door Vlaanderen on Wednesday in Belgium
Jelle Wallays makes a clever attack with one kilometer left, and Topsport teammate Theuns sprints out of the group to snatch second place
Big names and bigger teams are fine-tuning form and team compositions in this week's semi-classics ahead of the major spring races