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.
Among those missing their flight, La Libre reported, was former Belgian Prime Minister Yves Leterme.
"I've been standing in line for about the past hour, and whether I make my flight or not is an open question at this point," said one man.
On the airport's Facebook page some users described the situation as "ridiculous", "chaotic" and a "nightmare".
Others warned that the three-hour check-in time could deter business passengers.
But some were sympathetic. "At the moment it's fine, we know what happened and all. But for the future, it has to be faster," said one woman.
An airport spokeswoman said the situation should improve in the coming days.