Spaniards angered by increasingly grim economic prospects and unemployment hitting one out of every four citizens turned out in droves in the nation's largest cities.
The protest marked the one-year anniversary of a spontaneous movement that inspired similar anti-authority demonstrations across the world.
The Interior Ministry said 72,000 people marched against the government's tough austerity measures in Madrid, Barcelona and six other cities, but protesters claimed the turnout was much higher.
The epicentre of the protest was Madrid, where at least 30,000 people flooded into the central Puerta del Sol plaza last night, vowing to stay put for three days.
Authorities warned they would not allow anyone to camp out overnight as protesters did last year, but the demonstrators stayed put after a midnight deadline to leave and more than 2,000 riot police on duty made no immediate effort to force them out.
“I'm here to defend the rights that we're losing and for the young people who have it so tough,” said 57-year-old teacher Roberto Alonso. “They're better educated than ever. But they don't have work. They don't have anything. They're behind and they'll stay that way.”
At least 22,000 people demonstrated in Barcelona, Spain's second largest city. Marches were also held in Bilbao, Malaga and Seville, and sympathisers from other countries held protests across Europe.
The protests began May 15 last year and drew hundreds of thousands of people calling themselves the Indignant Movement. The demonstrations spread across Spain and Europe as anti-austerity sentiment grew.
Spain is in dire economic straits, prompting fears it may need a bailout similar to those requested by Greece, Ireland and Portugal. It is in recession, and unemployment stands at almost 25% - the highest among the 17 countries using the euro. One in two Spaniards under 25 are out of work.
Prime minister Mariano Rajoy's conservative government has enacted deep spending cuts to reduce the national debt, but many people blame those measures for deepening families' financial plight.
Javier Colilla, a 27-year old university student, said he protested in Madrid because Spain's economic situation seemed like it would spiral into chaos.
“We've had this crisis for four years, but it feels like it's just starting,” the fine arts major said.
Mr Colilla, who lives with his parents, said he saw no prospect of getting a menial job after graduation and thinks he may never be able to buy a flat.
“Right now I'm thinking my best option will be to go to Germany where I can wash dishes, make a little money and learn German,” he said. “The prospects of getting a job in Spain are practically inexistent.”
He said the government austerity cuts targeting “health and education, but rescuing banks are wrong. They need to find other places to cut”.
A year ago, the “indignados” pitched tents and occupied town and city squares across Spain for weeks. Demonstrators clashed with police who eventually moved in to evict them.
“We are here today to celebrate one year since the ... movement started and though we have achieved some things the situation is much worse now, so we need to keep fighting to get things better and that's why we are here today,” said 40-year-old activist Ana Pancorvo.
Antonio Barroso, a London-based Europe analyst for the Eurasia Group consulting firm, said the protests would probably “have no impact on the the (Spanish) government's strategy” to appease international investors by sticking with its controversial austerity drive.
Protests also took place in London and other European cities, and were planned in South American countries including Brazil and Chile.
In London, several hundred anti-capitalist protesters from the Occupy movement marched peacefully through London's financial district, rallying outside the offices of international banks.
Hundreds also took to the streets in Brussels and Lisbon, Portugal, but the turnout was lower than last year.
The protesters called for governments to enact measures including a global tax on financial transactions and ensure more democratic international financial bodies.
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