Anthony Barrett was in his room in the Sheraton Brussels Airport Hotel when a blast ripped through an airport terminal opposite.
Mr Barrett works for the Wales Audit Office and is in Belgium for a work conference.
He told ITV Cymru Wales: "I looked out of the window and I could see windows blown out of the terminal building, people fleeing from the building and at that point I saw at least one person lying on the ground."
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AMs have passed emergency legislation, asserting that after Brexit the Assembly should get all EU powers on devolved matters.