The inter-governmental conference launched Saturday by the Italian rotating presidency of the European Union is the official start of the final negotiation process to create a new constitution for the bloc.
The aim is to clarify who does what inside the EU and to ready the institutions to deal with a much larger union once 10 new members join the existing 15 EU states next May.
In a joint statement the leaders said an existing draft constitution, the fruit of 18 months of preliminary negotiations, was a "good base."
The Italian Prime Minister Silvio Berlusconi urged his colleagues to avoid a wholesale renegotiation of the draft constitutional text, adding that a difficult task is ahead if differences are to be resolved.
"It must represent the start of a Europe that will have strong common institutions capable of ensuring peace, security and prosperity to its citizens and capable of a strong economic development policy," said Mr. Berlusconi.
But many countries still have problems with the text. Smaller nations and those that are still to join feel the constitution still fails to respect the principle of equality among large and small countries.
Differences still remain over the voting system that should be used. Britain wants certain foreign and defense issues, taxation and social security to remain subject to national vetoes. And members are also in disagreement over whether the constitution should make explicit reference to God, Christianity or religion.
As discussions were underway, hundreds of anti-globalisation protesters clashed with riot police. Tear gas was fired after police were struck with stones and bottles as they attempted to maintain security. Various demonstrators and policemen were injured and dozens detained for the disruption.