A demonstrator in Madrid carries a banner reading, 'Spanish Prime Minister, Rajoy, When will you send us to a gas chamber?' (AP)

Tens of thousands of demonstrators have marched in dozens of Spanish cities to protest against sky-high unemployment, corruption scandals and what they say is the government's inefficient handling of the economy.

Many protesters carried placards critical of Prime Minister Mariano Rajoy's ruling Popular Party, which is immersed in a corruption investigation centred on former treasurer Luis Barcenas and alleged under-the-table payments to party officials while in opposition.

King Juan Carlos' son-in-law, Inaki Urdangarin, is also under investigation on suspicion of having embezzled several million dollars.

Rallies were organised in Madrid and 60 other cities by 150 organisations including trade unions representing the construction, car and television industries as well as police and health services. Police estimated some 20,000 people marched in the north-eastern port city of Barcelona, but authorities did not have figures for a large rally held in Madrid.

Unemployed protester Javier Alonso, 55, said the government's labour reform policies were destroying employment while not easing the country's slide into recession.

He said: "All they have achieved is to give employers greater facilities to fire workers and us 50-year-olds have been rewarded with cheap dismissals which have simply dumped us on the streets."

Spain's unemployment rate is at 26% and the economy is immersed in its second recession in three years, with many young graduates and qualified professionals emigrating to find jobs elsewhere.

Health care worker Isabel Montanes said she was protesting because the cuts were badly affecting those worst off in Spain's society.

She said: "They want to cut what most sustains a country, which is education and health care. The unemployment rate is so immense that young people believe they have no future here, and we are embittering their existence."

General Workers Union spokesman Candido Mendez said it was clear most people rejected the government's austerity policies, which he said were pushing many toward poverty and away from democracy.