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A passenger gestures at El Prat Llobregat airport, near Barcelona, on Saturday. ((Manu Fernandez/Associated Press))

Threatened with prison, nearly all of Spain's air traffic controllers scheduled to work returned Saturday, a day after they walked off the job and stranded hundreds of thousands of travellers.

Government officials declared a "state of alarm" after the wildcat strike began Friday evening. The never-before-used decree prompted 283 of the 295 controllers scheduled to report for duty Saturday to do so, Spain's civil aviation agency AENA said.

The order, issued by Prime Minister Jose Luis Rodriguez Zapatero, placed the country's 2,000-odd air traffic controllers under military authority, facing the threat of jail terms under the military penal code.

Zapatero acted under a constitutional clause reserved for national emergencies such as earthquakes or other breakdowns in essential public services.

Spanish airspace reopened and flights were resuming, but an estimated 600,000 people missed flights Friday or Saturday because of the walkout, the aviation agency said.

Called in sick

The controllers had failed to show up work or called in sick en masse to protest against changes in working conditions and pay. They do not have a legal right to strike.

Transport Minister Jose Blanco summoned union representatives to talk with government officials and apologized to angry travellers who spent the night sleeping at airports on what was supposed to be the start of a long holiday weekend.

"I want to apologize to the citizens who can't travel because of the air traffic controllers' blackmail and who are being used as hostages," he said. "The government will do everything in its power to face a situation that we consider extremely serious."

Spain's air traffic controllers have been in negotiations for a year with AENA over wages, working conditions and privileges. The dispute intensified in February after the government restricted overtime, cutting the average annual pay of controllers from about $464,000 US to about $265,000.

The controllers' union has complained for weeks that many members have already worked their maximum hours for all of 2010, and that all 2,000 are overworked and understaffed.

With files from The Associated Press