Spaniards furious at hardship and corruption scandals in the financial crisis vowed a mass "citizens' tide" of protests in cities across the country on Saturday.
A grouping of civil associations and protest movements called in a statement for non-violent demonstrations in Madrid, Barcelona and scores of other cities.
February 23 marks the anniversary of an attempted coup in 1981 by right-wing officers who tried to crush Spain's young democracy and restore military rule.
"We are facing a real financial coup, that is why we have chosen this date," Paco Segura, spokesman for one of the organising associations, Ecologists in Action, told a news conference.
"Today our democracy is also threatened, that is why we protest to demand a true democracy where people can decide their future," he added.
The manifesto of Citizens' Tide targets financial market pressure, "brutal cost-cutting policies" plus "corruption and loss of legitimacy of the institutions".
Like the "Indignants" movement that erupted in May 2011 and swamped central Madrid for weeks, the movement says it has no formal leadership.
It unites workers in various sectors who have held their own demonstrations over recent months, such as teachers, nurses, doctors, students, miners, with movements such as feminists, environmentalists and small political parties.
Spain has been seeing weekly protests against the spending cuts and tax hikes imposed by Prime Minister Mariano Rajoy's conservative government to slash the public deficit.
Public anger has been fanned over recent weeks by a corruption scandal in Rajoy's conservative Popular Party.
Newspapers alleged that Rajoy and other party members received irregular payments, which he and the party have denied.
The cuts are squeezing the public sector, while the current recession that started in late 2011 has shut down companies and thrown millions out of work, driving the unemployment rate above 26 percent.
The protestors blame the crisis largely on banks that took part on a building boom that crashed in 2008. The financial sector has received 41 billion euros ($54 billion) of eurozone aid to save it from collapse.
"We will not allow them to continue to attack our social rights with these savage cuts to feed, with the revenue that they generate, the avarice of the banking system," said Segura.
Rajoy defended his government's record during a state of the nation address in parliament on Wednesday.
"We have left behind us the constant threat of imminent disaster and we are starting to see the path for the future," he said.
In Madrid, the movement called on demonstrators to converge on Saturday evening on Plaza de Neptuno, near the lower house of the Spanish parliament -- scene of a huge protest in September that led to clashes with riot police.