Spain's prime minister has said his government may extend an emergency decree it has put in place to end a 24-hour wildcat air traffic controllers' strike.
Flights have returned to normal after a weekend of travel chaos which disrupted hundreds of thousands of journeys.
Officials said 250,000 people were hit by Saturday's walkout, amid a long-running dispute about working hours.
The emergency measures had not been seen since military rule.
The state of alert allows the government to arrest those who refuse to work.
"The government has issued a decree for a period of a state of alert to ensure normality," which is due to continue for 15 days, Prime Minister Jose Luis Rodriguez Zapatero said.
"Depending on how the situation develops, the government will take the decision to extend the measure, and will of course do it taking public opinion into consideration and in conjunction with political parties."
Threats of further strikes over the Christmas and New Year period would thus be quashed by the government, reports say.
The controllers' unsanctioned action began on 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 as Spain tries to cut its budget deficit.
However, correspondents say their disagreement has won little sympathy with the Spanish public.
Air traffic controllers in Spain are paid high salaries, an average of 200,000 euros each (£170,000) per year.
The airport authority, AENA, has opened disciplinary proceedings against 442 controllers.