Prime Minister David Cameron has arrived at the EU summit, insisting he will be battling for eurozone stability and the protection of British interests.
He said eurozone stability was good for the whole of Europe, including the UK, adding: "We've got to protect British interests and that's what we will be discussing."
Before leaving London he said he would have "no hesitation" in wielding Britain's veto to block an EU treaty change to resolve the eurozone crisis if it did not meet UK requirements.
Mr Cameron is coming under intense pressure from some elements of his own party to demand the return of powers from Brussels to Westminster as the price for acquiescence in a treaty, and to put any deal to the British people in a referendum.
Downing Street insists that no referendum is needed, as proposed changes will not transfer powers to Brussels, and Mr Cameron has said that he will ensure that any treaty includes safeguards to protect the City of London from European regulation.
Mr Cameron spoke briefly as he went into the start of what could turn out to be marathon negotiations - the eighth EU summit on the eurozone crisis this year.
He said: "These are important talks and we need to get that stability in the eurozone. It's good for Europe and good for Britain. We've got to protect British interests and that's what we will be discussing."
His first meeting before a dinner with all 26 other EU leaders was due to be a private chat with Italian Prime Minister Mario Monti, brought in as a technocratic caretaker leader after the fall of Silvio Berlusconi.
French President Nicolas Sarkozy and German Chancellor Angela Merkel called for swift action when they addressed members of the centre-right European People's Party in Marseille ahead of the summit opening.
They have made clear that if they cannot secure support for a treaty change from all 27 EU members in Brussels, they are ready to press ahead with an accord limited to the 17 countries which use the single currency.