Police clash with protesters in Spain.

Tens of thousands of protesters have swamped Spain's streets to support a general strike, venting their anger over labour reforms and deep spending cuts, ABC News reported.

Thursday's strike was called to protest against strict new labour laws and more austerity measures, and nearly 1 million people took part in Madrid alone.

While the demonstrations were mostly peaceful, more than 170 people were arrested across the country and violence erupted in parts of Barcelona and Madrid.

In Barcelona, thousands of picketers tried to stop businesses opening and clashed with police outside banks and stock exchanges in noisy confrontations.

Police shot smoke canisters and fired rubber bullets into the ground so they would ricochet into people's legs.

An interior ministry spokesman said a Starbuck's coffee shop and rubbish bins were set alight, blaming the clashes on a small group of people who were "provoking confrontations and violent incidents".

The strikers are angry about employment changes making it easier and cheaper to fire workers.

They are also bracing for billions of dollars in spending cuts and tax rises expected in the government's budget.

The Spanish government says that is necessary for the country's economy to improve and to meet strict debt targets set by the European Union.

Spain has an unemployment rate of 23 per cent - the highest in the European Union - with half of all people under 25 jobless.

The country is tipping into a second recession, and the European Union is demanding it meet targets to reduce its debt.

Airports were affected with hundreds of international flights postponed and train and bus services were cut during the strikes.

Many commuters still crowded train stations and bus stops, saying they could not afford to lose a day's pay.

At a union rally in central Madrid, 56-year-old civil service worker Angel Escolar said the labour reform removed basic workers' rights.

"They can change your hours, change your shifts, cut your salary and lay off people without justifying it," he said.

"They are going to throw us out and hire young people for less and with worse conditions."

Thursday's strikes are the first to challenge prime minister Mariano Rajoy, who was sworn in 100 days ago vowing to fix Spain's faltering economy.

They came a day before the conservative government adopts the new budget, adding to cuts that have already squeezed public services.

A spokesman for the Catalan government, Francesc Homs, said a minority had taken advantage of the protests.

"I don't think this is linked in any way to the unions," he said.

"We are sorry that they have ended up becoming the protagonists of this day."