In the capital Madrid, participants brandishing banners reading "for a democratic rebellion by European peoples, sovereignty, dignity, solidarity," converged on central Sol Square.
The march was the latest in a series of protests which began on March 22 and has gathered support from around the country.
More than 125 political organisations, unions and social groups had urged people to take to the streets of dozens of cities including Seville and Barcelona to "show their indignation" over economic hardship.
Spain's political class has struggled to rally popular support in a country weary of years of austerity -- although Madrid did win a reprieve last week from Brussels on fines for overshooting its fiscal targets.
The political fallout from the eurozone crisis saw no party win a majority in December voting, and a second election inside six months will follow in June after all attempts to form a coalition foundered.
The ruling right-wing Popular Party topped the December poll but fell short of an absolute parliamentary majority as two new upstarts -- far-left Podemos and centrist Ciudadanos -- took votes away from the mainstream conservatives and Socialists.
Former mechanic Fedor Asuar, 81, was among the demonstrators in Madrid, waving a banner reading "impoverished grandparents, mortgaged children, unemployed grandchildren."
He said he was marching to "defend my tiny pension" and to protest at "a series of swingeing cuts in education, health..."
Protesters said they feared that the June 26 poll will hand Spain a government which will continue making cuts to public services even at a time when around one in five Spaniards are living close to the poverty line, on barely 8,000 euros ($8,400) a year.
Unemployment is running at around 21 percent -- second only in Europe to debt-laden fellow EU struggler Greece.
Marchers also directed their ire at proposals to create a Transatlantic free trade framework between EU states and the United States, and spoke in favour of more humane rules governing asylum for migrants.
Yayo Herrero, a 50-year-old teacher, read out a declaration blaming the banks and the European Union for "taking away sovereignty of the countries" in the bloc" with governments forced as a result to follow "policies that go against peoples' interests and serve capital."