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Heathrow Airport has said 39 flights - nearly all involving Spain or Portugal - had been cancelled today at the west London airport, as a result of strike action across Europe.
Air passengers endured travel misery today as UK airlines cancelled flights due to anti-austerity strikes across Europe.
British Airways had to axe eight round-trip flights between London and Madrid and Barcelona, as well as two return services to Lisbon in Portugal.
EasyJet cancelled some UK-Spanish services as well as more than 20 within-Europe flights.
Some other easyJet flights were badly delayed - with a London service to Thessalonaki in Greece delayed for three hours 40 minutes.
Passengers on an easyJet London to Athens flight faced a delay of two hours 30 minutes, while a Manchester-Athens flight was delayed for two hours 10 minutes.
Heathrow and Gatwick were among the UK airports where flights were delayed or cancelled.
The interior ministry said 32 people had been arrested and 15 people treated for minor injuries following disturbances in Spain, as strikes and protests against austerity measures took place across Europe.
The General Workers Union said the stoppage, the second in Spain this year, was being heeded by nearly 100% of workers in the motor, energy, shipbuilding and construction industries.
But ministry official Cristina Diaz played down participation, saying it was hardly noticeable in goods haulage and major city wholesale food markets.
She said electricity consumption, a gauge of industrial activity, was down 11% on a normal working day.
With a 25% unemployment rate, Spain is at the core of Europe's debt crisis.
Strikers have been arrested in Madrid, as a series of coordinated anti-austerity strikes take place across Europe. Union-organised action in Spain and Portugal is expected to be followed by demonstrations later in Italy, Malta, Cyprus and Greece.
Transport is expected to be disrupted, schools and public services closed and rallies held across the continent to protest over spending cuts, tax hikes and workers' rights.
Workers in Spain and Portugal are taking part in general strikes today in protest against anti-austerity policies being implemented by governments in both countries.
European strike action, which has been urged by the European Trade Union Confederation (ETUC), has lead to five departures and five arrivals being axed at Gatwick throughout today.
A spokesman for the airport said services between the UK and Lisbon, Porto and Madrid were affected.
Heathrow Airport said 12 flights have been cancelled as a result of the austerity strikes."British Airways flights travelling both to and from the continent are affected," a spokesman added."Passengers due to travel today are advised to contact their airlines for the information."
Southampton, Luton, Stansted and Birmingham airports said no services are yet to be affected by the strikes.
Travellers flying into and out of the UK are facing disruption as workers across Europe prepare to stage a series of strikes against rising unemployment and austerity measures.
UK airports said a number of flights to destinations across the continent had been cancelled, which is expected to involve around 40 groups from 23 countries.
Strikes in Spain, Greece, Portugal and Italy have been planned as part of The Day of Action and Solidarity across the EU.
Protests are also expected in Belgium, Germany, France the UK and some eastern EU states.
Anti-austerity strikes have begun across Europe.
Union-organised action began in Spain, Belgium and Portugal, with similar demonstrations expected to take place later in Italy, Malta, Cyprus and Greece.
Transport was expected to be disrupted, schools and public services closed and rallies held across the continent to protest over spending cuts, tax hikes and workers' rights.
Latest ITV News reports
Police and protesters clashed in Spain, Italy and Portugal as millions of workers went on strike to challenge to austerity policies.
As well as strikes across Europe, today has also produced figures that may convince people at the top that austerity is self-defeating.