The officers are protesting against government reforms and staff shortages.
Trades unionist Marc Nijs explained: "At the minute very few people are being recruited, while many colleagues are retiring. Here at Zaventem we work around the clock. A minimum number of staff is required in order for customs to function properly. It's becoming very difficult if places systematically remain unfilled. The result is fewer checks. Without the necessary number of checks we can't guarantee security."