President Jacques Chirac has taken part in a live TV debate in France to try to persuade its people to vote in favour of the proposed EU constitution.
Chirac was trying to jump-start the flagging Yes campaign
Mr Chirac said the treaty was needed to keep the EU "strong and organised" and defend its interests against the power of the US or of India and China.
He was trying to jump-start a flagging Yes campaign in the referendum on the text due to be held on 29 May.
Recent opinion polls suggest a majority will vote against the constitution.
Mr Chirac's critics accused him of being afraid to face serious interrogation, but he faced tough questions nonetheless from a hand-picked audience of 83 young people, hosted by popular talk show hosts rather than heavyweight political interviewers, says the BBC's Allan Little in Paris.
Mr Chirac warned a No vote against the new constitution - designed to streamline institutions to make decision-making easier in an enlarged union of 25 countries - would be a disaster for Europe.
It would halt the European project in its tracks, and pave the way to an unregulated, uncontrolled free-market world, dominated by the United States.
He said it would be in the interest of Anglo-Saxon countries or the US to stop "European construction" and that France would be weakened if it voted No.
Mr Chirac said there would be no room for renegotiation if France voted against the constitution, and that the country would be seen as "the black sheep" of Europe "that blocked everything".
The French president - who has staked his prestige on the constitution being approved - ruled out his resignation should voters reject the treaty.
He added that Prime Minister Jean-Pierre Raffarin's leadership was not at stake either.
"We are not asking a question about who governs us for the next five years," he said.
He agreed that the French seemed ill-informed on the issues.
Around half of respondents on the panel of people aged 18 to 30 taking part in the debate said they did not understand the text which they found too complex and that they would vote no as a result.
Many expressed worries about the new EU being too committed to free-market economics and threatening France's social model.
But Mr Chirac said the European treaty answered to "a logic that is indeed not liberal".
He urged France not to be afraid of the new text, stressing that public services remained protected by national governments and did not fall under the competence of the EU.
Mr Chirac said Europe was the world's first commercial power, the first exporting power, the first investing power, the first sporting power, the first in providing aid to developing countries - "a generous Europe".
On the question of Turkey, he said its possible entry into the EU was a separate issue from the new EU constitution.
"What is a problem is that today Turkey's values, traditions, lifestyle and the way it functions are incompatible with the values of Europe. That much is clear," he said.
All 25 EU nations must ratify the new treaty before it can take effect.
It has already been approved by Spain, Slovenia, Lithuania, Hungary and Italy, while Greece is expected to follow suit in a parliamentary vote on Friday.
However, French rejection would almost certainly kill off the treaty, correspondents say.
If France votes No, the damage to Mr Chirac's presidency would be significant - and the damage to France's standing in the EU would be incalculable, our correspondent says.