ThinkSpain reported the protest in Barcelona was the city's biggest May Day demonstrations in years, with participants hoisting banners and posters reading "Work, Rights and Dignity." Estimates of the crowd size ranged from 15,000 by the Catalonia police force Mossos d'Esquadra to about 100,000 by trade union leaders.
No incidents were reported.
Thousands of citizens also took part in demonstrations in Madrid, Lleida, Girona, Tarragona and Tortosa, ThinkSpain said.
The May Day protests were the first in a series of demonstrations planned for at least the next two weeks, demanding radical political and economic changes from a government protesters claim is not simply deaf but defiantly ignoring their calls for a totally different approach -- cutting funding for the Catholic Church, for example, rather than for education and healthcare.
The next protest is expected in Barcelona Thursday, when the European Central Bank is to hold its governing council meeting in Spain's second-largest city.
The government of Prime Minister Mariano Rajoy has announced further austerity measures even as it reported Monday the economy contracted 0.3 percent each of the past two quarters and said Friday unemployment had reached to 24.4 percent.
More than half of Spaniards under 25 are jobless, the government said.
Retail sales fell for a 21st straight month, down 3.7 percent in March.
"There's no contradiction between budget cuts and economic growth," Spanish Finance Minister Luis de Guindos said Monday. "Budget cuts are essential to create the conditions to finance growth."
The argument used by Spain and other struggling European economies is that if they continue pursuing policies of austerity, even as growth collapses and job losses mount, they will be rewarded by investors' greater willingness to buy their bonds, The New York Times said.
This has not happened yet.
Standard & Poor's downgraded Spain's sovereign-debt rating two notches Thursday and lowered the ratings of 16 Spanish banks Monday.
Spain's deep austerity drive includes tens of billions in spending cuts to bring the budget deficit down to 3 percent of gross domestic product next year from 8.5 percent of GDP in 2011.
Rajoy announced last month Spain would not meet its deficit-reduction goals for 2012.
Spain said Monday the latest spending cuts would include an additional $13.25 billion extracted from regional and local governments, bringing the total to about $27 billion this year.
The measures consist of lower spending on public services such as healthcare and education -- two of the core issues the demonstrators are protesting.
"People are angry that budgets for fundamental constitutional rights to healthcare and education are being slashed, while budgets for the Catholic Church and defense are all but untouched," Maria Elena Ferrer, a political author and principal of the non-partisan Humanamente consulting firm of Valencia, Spain, and suburban New York, told United Press International Monday night.
She pointed out that as tens of thousands of people across Spain protested Sunday against the government's education and healthcare cuts, "Rajoy defiantly countered he would continue his unpopular austerity measures until the end of his term."
Rajoy told the ruling Popular Party's Madrid regional congress Sunday, "There will be reforms announced this Friday, and every Friday after that, and they will be major reforms.
"A lot of people cannot understand the decisions that I am taking at the moment," Rajoy said. "But the problem is the crisis, unemployment, the recession and disordered public finances. We have to make structural changes and to take root and branch measures."
Additional anti-austerity protests are planned through at least mid-month, when activists promise to observe the anniversary of the "indignados," or indignants, protests, which started May 15, 2011.
Since then, Spain's protests have brought 6.5 million to 8 million Spaniards to the streets, Spanish public broadcaster RTVE reported.