Demonstrators are angry at the government's economic policies and Spain's high youth unemployment rate.

Their numbers have swelled despite a ban on political protests ahead of elections.

The governing Socialists are expected to suffer major losses in voting for city councils and regional governments.

Prime Minister Jose Luis Rodriguez Zapatero's government is struggling to overcome recession and create jobs.

In the capital Madrid, about 30,000 people have occupied the central Puerta del Sol square.

Similar protests, popularly known as M-15, have sprung up in many other cities including Barcelona, Valencia, Seville and Bilbao.

"I'm happy that they're finally protesting. It was about time," said Maria, an elderly woman visiting her grandson in the Puerta del Sol.

Protest organisers have urged those taking part not to confront the police, and have tried to discourage the distribution of alcohol.

"It's a revolution, not a drinking party," read one sign.

Brooms donated by supporters are being used to keep the square clean, witnesses say.

The BBC's Sarah Rainsford in Madrid says there is also a creche, a kitchen area and even a vegetable patch.

One protester, Alejandro, said: "I hope this changes our situation. We have a right to regular jobs, a future and a decent salary, to more opportunities in life, the chance to get a house, to pay for that house without being enslaved, but especially a better quality of life."

The protesters are urging people not to vote for either of Spain's two main parties - the governing Socialists or the centre-right opposition.