Trade unions in Spain, Portugal, Italy, France and Greece used the traditional marches to express anger over a savings drive across the eurozone, aimed at shoring up public finances but criticised for forcing countries deeper into recession.
Italian demonstrators briefly clashed with police in riot gear in Turin and thousands marched in the central city of Rieti to listen to the leaders of the country's three main unions denounce Prime Minister Mario Monti's reforms.
French trade unions organised about 290 demonstrations, from Marseille in the south to Strasbourg in the east, as well as in Paris. The Interior Ministry said 316,000 people turned out altogether, compared to 77,000 in 2011.
President Nicolas Sarkozy attracted almost 100,000 to a rival Paris rally for "real workers" after the largest union, the CGT, advised members to vote him out of power on 6 May, the first time a union has openly urged a vote against a candidate.
Far right blank vote
French voters seem poised to choose policies favouring economic growth over austerity with Socialist François Hollande leading Sarkozy in the polls. Far-right leader Marine Le Pen, whose supporters are key for the result, told her party's annual rally on Tuesday she would cast a blank vote.
Le Pen came in third in the first round of the elections on 22 April, getting 17.9% of votes. Recovering a part of her electorate in the 6 May runoff is important especially for Sarkozy, who is trailing in opinion polls.
In Madrid, tens of thousands headed in the rain to the main square waving signs opposing cuts, while thousands turned out in Lisbon. In Athens around 5,000 workers, pensioners and students marched with banners reading "Revolt now" and "Tax the rich".
Greece will vote on Sunday in a parliamentary election that risks derailing the international bailout keeping the country afloat by punishing the parties that backed the package.
"Our message will be stronger on Sunday," said Maria Drakaki, 45, a public-sector worker whose salary has been cut.
"There's no way I'm voting for one of the two main parties."
The two biggest Greek parties, the Socialist PASOK and the conservative New Democracy, have ruled Greece for decades but are expected to struggle to win enough support to renew their pro-bailout coalition.
Spain's jobless rate rose to near 25% in the first quarter, more than double the EU average, as the economy sank into recession. Some economists, including those at the International Monetary Fund, have questioned whether deep cuts should be made at the expense of growth.
Portuguese hit the streets
In Portugal, thousands of people rallied in Lisbon, some with placards saying "Stop the robbery, no more stolen wages".
The 700,000-strong CGTP union, which refused to sign a pact on labour market reforms required under a €78-billion EU/IMF bailout, rallied across Portugal under the slogan "Against exploitation and impoverishment, for a policy change".
"Austerity is not a solution for Portugal or Europe," said João Proença, chief of the UGT union, the second biggest. "The pivotal issue is to promote job creation."
Portugal is implementing tough austerity measures, which have deepened its recession and pushed unemployment to all-time highs of around 15%.