Spain is in deep economic difficulty, prompting fears it may need a bailout similar to those helping Greece, Ireland and Portugal. It is in recession, and unemployment stands at almost 25 per cent — the highest among the 17 countries using the euro. One in two Spaniards under the age of 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.
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.
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.
The mostly young protestors aim to occupy central Puerta del Sol plaza in the Spanish capital Saturday evening and stay for three days. But authorities have warned they won't allow anyone to camp out overnight, and up to 2,000 riot police will be on duty.
"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 who was hooking up with one of four Madrid marches due to converge on the Puerta del Sol.
In Britain, about 200 anti-capitalist protesters from the Occupy movement marched through the English capital's financial district, rallying outside the offices of major banking groups such as Merrill Lynch and Santander. Protests were also planned in other European cities.
Antonio Barroso, a London-based Europe analyst for Eurasia Group, said he doubted the Spanish protests would force Rajoy's government to change its policies.
The demonstrations "will probably have no impact on the government's strategy," he said in a written analysis.