The military in Spain has taken over air traffic control in the country after civilian traffic controllers called in sick en masse.
The move - authorised by the government - came after several airports were shut on Friday after the unauthorised walkout.
Most of Spain's airspace remains closed.
The BBC reports the controllers are involved in a dispute with the AENA airport authority over hours and conditions.
The government has accused the controllers of holding the country to ransom. It says it cannot accede to their pay demands in these times of austerity.
The strike began at 5pm local time (1600 GMT). More than 100,000 passengers were hit at the start of a long weekend.
Barajas airport in Madrid was one of the first to be affected, along with airports on Majorca, Ibiza and Menorca in the Balearic Islands. Barajas is the busiest airport in Spain.