Spanish Prime Minister Jose Luis Rodriguez Zapatero has voted in the referendum on the EU Constitution.
Spaniards are the first of ten nations to have their say on the text in a popular ballot.
Both the country’s ruling Socialists and the right-wing opposition back the charter, as do most Spaniards, according to opinion polls.
But the big question is whether turnout will be high enough to give a “yes” vote credibility.
“This poll could pave the way for a stronger, more united, and more prosperous Europe,” Zapatero told reporters after casting his ballot.
“To achieve that, I am calling on all my fellow citizens to take part in the vote.”
Opinion polls have shown that only 40 to 50 percent of Spaniards are likely to participate.
The Constitution aims to streamline decision-making as the European Union expands but, in Spain as in other countries, most people say they have little idea about its content.