Conservative leader Nicolas Sarkozy, a former interior minister renowned for his tough stance on immigration and security, said the EU could not be expanded further until after a major overhaul of its institutions, and spoke firmly against granting membership to Turkey.
However, he said his opposition was not politically or ideologically motivated, but stemmed from the country's geographical position.
"Turkey is situated in Asia Minor," he said.
Socialist Segolene Royal agreed that the EU's founding members should take a break from further enlarging the 27-nation club, but said Turkey could eventually become a member, provided that it meets all accession requirements.
However, any new proposal on EU expansion should be put up for a national referendum in France, she said.
Royal also spoke for a fresh plebiscite on the European Constitution, overwhelmingly rejected by the French in a May 2005 referendum.
Her rightist opponent dismissed the idea, saying the Constitution should be replaced with a simpler constitutional treaty, whose adoption requires nothing but parliament's approval.
Wednesday night's 160-minute duel, broadcast live on two major French TV channels, was dominated by domestic issues, such as unemployment, national debt, street crime and immigration.
Opinion polls suggest the runoff will be too close to call, but predict a victory for Sarkozy, who topped the first round ballot April 22, garnering 31.2% of the vote. Royal came second with 25.9%.
