The leaders emerged from a four-hour summit in Rome promising the constitution will take effect in 2005. By then, 10 more nations will have joined the bloc, raising the EU membership to 25.

But tough bargaining lies ahead in the next 10 weeks.

As the leaders left the summit Saturday, their foreign ministers began the first of a series of debates to settle disagreements over the text.

"We have opened a new page in Europe's history," French President Jacques Chirac said.

Chirac and other leaders cautioned against a wholesale re-negotiation of the draft.

The constitution foresees an EU president, a foreign minister, a structured defense policy and provisions to make it more difficult to wield vetoes that cause bureaucratic gridlock. It also calls for an EU executive of only 15 members, denying each state the automatic right to one European Commissioner.

Italian Premier Silvio Berlusconi, whose country holds the EU presidency, said "a very difficult task awaits us" in overcoming differences.

British Foreign Secretary Jack Straw suggested that divisions over the war in Iraq should not stop EU nations from seeking an integrated security and foreign policy, saying "we are stronger when we are united."

The leaders met amid heavy security at the Palazzo dei Congressi, a Fascist-era convention complex in southern Rome, away from the city's historic centre.

Police cordoned off a 2 1/2-mile zone around the venue to guard against thousands of anti-globalisation protesters, who swung wooden clubs at a line of riot police. Authorities responded with tear gas.

After a standoff around the summit site in a southern Roman suburb, several thousand marchers tried to push past a thick line of police. They hurled eggs and rotten fruit and then used a large shield to try to breach the barrier, shouting: "Murderers! Murderers!"

About 6,000 people took part in the anti-globalisation protest in the southern suburb of EUR, police said, while 15,000 others gathered for a calm demonstration in the city centre. Protesters elsewhere gutted an employment agency, vandalised two gas stations and a bank, and threw rolls of toilet paper at police. Authorities detained about 50 demonstrators.