Turnout of 42 percent, however, was the lowest in Spain's modern democratic era. Only a third of eligible voters endorsed the constitution with their ballot, just enough for Zapatero to avoid political embarrassment.
Supporters of the charter had hoped Spain would start a snowball effect to sweep away doubts elsewhere as ratification is needed by all 25 member countries. The EU has become synonymous with prosperity in Spain since it joined in 1986 thanks to 86 billion euros in EU funding.
"The abstention may set warning lights flashing in some countries because we were trying not just to export a "Yes" vote but also to export enthusiasm," said Jose Ignacio Torreblanca of Madrid's Real Instituto Elcano.
"There is a communication problem between the political class, which regard this treaty as fundamental, and ordinary citizens who do not share its importance," he told state radio.
EU leaders hailed the result as encouragement to other members and expressed confidence the charter would be approved. Nine other countries are due to hold referendums, with the toughest test coming when eurosceptic Britons vote in 2006.
"Voter turnout continues to be a challenge," EU Commission Vice President Margot Wallstrom told a news conference. "Governments, parliamentarians and the European institutions should step up the campaign to inform about the constitution."
British Foreign Secretary Jack Straw, speaking in Belgium before an EU meeting, hailed the Spanish result, adding: "I think it will help to reassure the electorate in Britain this really is in the interests of all the EU member states."
With EU political heavyweight France due to vote by July, a wave of strikes and protests against President Jacques Chirac's reforms have sown fears voters will use a referendum to punish his government.
"This 'Yes' of the people of Spain ... is a strong symbol and it shows the way to the other countries that will ratify this treaty in the coming months," Chirac said in a letter to Zapatero.
Analysts had said a strong endorsement by Socialist Spain could help Chirac to dispel left-wing criticism of the charter at home, but French media on Monday lambasted Spanish apathy.
In a front-page headline, French daily Le Figaro said: "Spain: The passionless Yes".
"This first 'yes' is unambiguous, but it is disappointing," the paper said in an editorial. "For Spain remains fundamentally europhile and could have expressed this support more massively towards more sceptical countries."
French Prime Minister Jean-Pierre Raffarin pledged to use the months before France's referendum to inform voters "more completely" about the constitution.
In Spain, conservative opposition leader Mariano Rajoy criticised Zapatero for rushing Spaniards to the polls without enough information on the charter.
"We're not going to be a model of anything," he said, rejecting government claims his party had been unenthusiastic in its campaign in favour for the constitution.
Observers said the predictability of a "Yes" vote and the lack of opposition may have dissuaded Spaniards from voting. Many Spaniards also stayed away because the referendum was not legally binding as parliamentary approval is still required.