Long delays have marred the reopening of the departure hall at Brussels airport weeks after suicide attacks there killed 16 people.
Amid heavy security, some passengers complained of missing their flights.
They have been advised to arrive at least three hours ahead of departure.
The 22 March attacks, by so-called Islamic State (IS), severely damaged the hall. Another suicide blast on a metro train on the same day killed another 16 people.
On Sunday some passengers were able to board select flights and from Monday could check-in at more than 100 desks in the airport departure hall.
It is not expected to be operating at full capacity until mid-June.
But the extra security checks and screenings have caused frustration, with local media reporting two-hour long queues outside the hall.