A wildcat strike by Spain's air traffic controllers has grounded flights across much of the nation, stranding hundreds of thousands of travellers.
About half of the controllers showed for their shift on Saturday morning but most refused to work, in a dispute over hours and conditions.
The government is meeting to decide whether to declare a "state of alert" and compel controllers to work.
National carrier Iberia has cancelled all flights until 0500 GMT on Sunday.
There are huge crowds of passengers at Spain's airports, many hoping to get away at the start of a national holiday, many of them frustrated and angry, says the BBC's Sarah Rainsford in Madrid.
The army was called in to take charge of the country's air space on Friday, but can not direct air traffic.
Some flights were operating to parts of Spain, including the Canary Islands and Majorca but flagship carrier Iberia, and budget airline Ryanair said they were cancelling all their flights until Sunday morning.
Deputy Prime Minister Alfredo Rubalcaba threatened legal action against the controllers if they did not return to work.
"If the situation doesn't normalise, the government will declare a state of alert," he said early on Saturday.
"The controllers will be mobilised and if they don't get back to work, their cases will be passed immediately to the judiciary and they will be accused of a crime which could mean a prison sentence."
The controllers' unsanctioned action began Friday afternoon in Madrid, with staff calling in sick.
It spread across the nation, forcing travellers to find last-minute hotel rooms or sleep on airport floors. Some passengers were taken by coach to their destinations.
The controllers were already involved in a dispute about their working hours, but were further angered by austerity measures passed by the government on Friday which would partially privatise AENA.
"We have reached our limit mentally with the new decree approved this morning obliging us to work more hours," said Jorge Ontiveros, a spokesman for the Syndicate Union of Air Controllers.
"We took the decision individually, which then spread to other colleagues who stopped work because they cannot carry on like this. In this situation we cannot control planes."
The head of AENA, Juan Ignacio Lema, said the strike was "intolerable", and told the controllers to "stop blackmailing the Spanish people".
Spanish Transport Minister Jose Blanco has also condemned the strike, saying those involved were "using citizens as hostages".
Hundreds of national and international flights have been cancelled across the country, leaving angry passengers left stranded in airports.
Some were left stranded on runways as their planes had to turn back. Others had to travel by bus to regional destinations.
"All flights are blocked, there's a huge lot of people here, sitting around everywhere. Right now everyone is calm, but we don't know what's happening," said one traveller at Barajas airport.
"The captain came out to say Spanish airspace had suddenly shut, with no prior warning," another passenger stuck in a plane at Palma told Spanish radio.
One woman at Barajas airport said it was "a disgrace". "How can a group of people be so selfish as to wreck the plans of so many people?"