Key European leaders have urged a meeting of European Union member states not to get bogged down in negotiations on a new constitution.
Demonstrators and police clashed as the meeting got under way
Italian Prime Minister Silvio Berlusconi welcomed delegates to the start of talks in Rome to finalise the EU's first constitution, saying he was optimistic that talks could finish on time before the end of the year.
But in a sign of potential difficulties to come, Spanish Prime Minister Jose Maria Aznar said he would not bend in his opposition to proposed new voting rules.
However, Britain warned the conference that failure was not an option and France urged member states not to revisit issues already thrashed out by an 16-month-long convention which drew up a draft version of the constitution.
As the leaders from the 25 current and future states gathered amid high security at
Rome's convention centre, anti-globalisation protesters clashed with police.
Police fired teargas as demonstrators armed with sticks forced them to cower behind riot shields.
'Act of will'
Saturday's meeting was the official start of the negotiating process, known as an intergovernmental conference, to create a new treaty or constitution for the EU.
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.
Mr Berlusconi said that, whereas the birth of the post-war European Economic Community had been an "act of faith", EU states would now need an "act of will" to draw their interests together.
But each of the countries has - to a lesser or greater degree - their own complaints about the draft constitution drawn up by the Convention on the Future of Europe, headed by former French President Valery Giscard d'Estaing.
That convention struck a delicate balance between competing interests and French President Jacques Chirac told the conference that, while not perfect, the draft should be accepted fairly much as it is.
"To contest this or that element of the compromise would inevitably open a Pandora's box and could lead to the failure of the intergovernmental conference," he said.
But Mr Aznar indicated there would be no backing down on Spain's particular gripe - shared with Poland - about the way votes will be shared out in the 25-member union.
He said he would not be swayed by warnings from France and Germany would be starved of cash if it did not show flexibility.
"We are no longer a backward, lagging country as we were before. We are a prosperous country and that has far-reaching consequences on how we approach these political questions," he said.
But there were sharp words for Poland's position - one of the most assertive future members.
"We know well the agriculture problems that Poland has to contend with, we're going to help them a lot but they must understand that they can't have their cake and eat it too," said Belgium's Foreign Minister Louis Michel.
UK Foreign Secretary Jack Straw said that Britain broadly accepted the text.
"I'm not contemplating failure here. We want to see improvements and that is why we are going into these negotiations," he said.
Key changes proposed by Mr Giscard d'Estaing's draft are:
- Governments will be less able to wield a national veto
- The European Parliament will have more power
There will be an EU foreign minister
- The European Commission - the executive arm of the EU - will lose some commissioners.
Germany, France, Italy, the Netherlands, Belgium and Luxembourg, the UK and Denmark are largely happy with the draft.
But many of the smaller members are unhappy about the proposal to cut the number of EU commissioners to 15, fearing a loss of influence. Currently there are 20 Commissioners, with a total of 25 when the new members join.
Members are also in disagreement over whether the constitution should make explicit reference to God, Christianity or religion.
Some Roman Catholic countries, notably Poland, Spain and Italy are demanding that religion is noted, but France is strongly opposed.