European Union leaders are renewing their vows on the 60th anniversary of the troubled bloc's founding treaties at a special summit in Rome designed to show unity despite Britain's looming divorce.

Meeting on Saturday in the same Renaissance-era palace where six founding countries signed the Treaty of Rome on March 25, 1957, the 27 leaders, minus Britain, will endorse a declaration of intent for the next decade.

The ceremonial gathering in Rome began with an audience with Pope Francis who gave messages of solidarity in the Vatican on Friday evening.

"You are called on to blaze the path of a new European humanism," Francis told the leaders.

"When a body loses its sense of direction and is no longer able to look ahead, it experiences a regression and, in the long run, risks dying," he said.

The White House congratulated the EU overnight on its 60th birthday, in a notable shift in tone for President Donald Trump 's administration, whose deep scepticism about the bloc has alarmed Brussels.

However, British Prime Minister Theresa May's absence from the summit, four days before she launches the two-year Brexit process, and an argument over the wording of the Rome declaration underscore the challenges the EU faces.

High security

Security is tight with snipers on rooftops, drones in the skies and 3,000 police officers on the streets, as Italy takes no risks following an attack this week in London claimed by the Islamic State of Iraq and the Levant ( ISIL , also known as ISIS) group.

Around 30,000 protesters are expected to take part in four separate marches - both pro- and anti-Europe - throughout the day in the Italian capital. Police plan to stop all traffic and declare a no-fly zone.

The Rome Declaration that the leaders will sign proclaims that "Europe is our common future", according to a copy obtained by the AFP news agency, after a series of crises that have shaken its foundations.

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Mass migration, the eurozone debt crisis, attacks by armed groups and the rise of populist parties have left the bloc, formed from the ashes of World War II, searching for new answers.

Yet, the leaders are deeply divided over the way forward almost before they have started.

Polish Prime Minister Beata Szydlo only agreed to sign the declaration at the last minute, after bitterly opposing a reference to a "multi-speed" Europe favoured by powerhouse states France and Germany , AFP said.

Poland , central Europe's largest economy, is concerned that, as one of nine of the EU's current 28 members outside the eurozone, it could be left behind should countries sharing the single currency push ahead with integration.

Greece , the loudest voice against the austerity policies wrought by its three eurozone bailouts, meanwhile insisted that the document should mention social policies.