EU leaders gather to mark 60 years since the Treaty of Rome

Mark Stone, Europe Correspondent

Parts of central Rome are in lockdown this weekend as Europe's leaders gather in the Italian capital to mark 60 years since the signing of a treaty which led to the formation of the European Union.

With the notable exception of the UK, presidents and prime ministers from all EU countries are celebrating the anniversary of the Treaty of Rome.

In a complex security operation, cars have been banned from the city centre and some museums and other tourist attractions have been closed.

The anniversary comes at an acutely uncomfortable time for the EU.

On Wednesday, the UK will become the first country ever to formally signal its intention to abandon the union, when it triggers Article 50.

:: What happens when Article 50 is triggered?

The bloc also faces an existential challenge from migration and a deep, simmering financial crisis, both of which have precipitated a rise in Eurosceptic nationalism across the continent.

"This will be an opportunity to reflect on the state of the European Union and the future of the integration process," says the leaders' invitation letter from European Council president Donald Tusk.

"It is no secret that the historical moment we are facing requires deeper and more solid reflection on the challenges for the Union," it says.

There was never any real expectation that the UK would attend the celebrations given that it is leaving the club.

:: Your guide to Brexit jargon

The remaining 27 leaders are using the moment and the grand backdrop of Rome to present a vision of the EU without Britain; to reaffirm their vows.

The morning of ceremony began in the Orazi and Curiazi Hall, where the original treaties were signed.

The presidents of the three EU institutions: the European Parliament, the Commission and the Council as well as words from the Italian prime minister and the prime minister of Malta, which holds the current presidency of the EU, will speak.

Plenty of reflection on the achievements of the European Project from its beginnings as the European Economic Community of just five countries to an integrated Union of 28 (soon to be 27) nations is expected.

The leaders will acknowledge current challenges and set out a vision of their priorities for the next 10 years with the publication of a 'Rome Declaration'.

Earlier this month, the European Commission published a white paper on the future of Europe.

In it, the Commission outlined five potential scenarios to drive the Union forward:

:: "Carrying On" (which essentially means the status quo - deepening of single market and more military integration)

:: "Nothing but the single market" (which does what it says on the tin)

:: "Those who want to do more, do more" (essentially a multi-speed Europe: some countries can integrate further without all having to follow)

:: "Doing less more efficiently" (a row back on status quo and a focus only on limited number of policies)

:: "Doing much more together" (a full throttle political union)

The white paper has formed a starting point for visions on the EU's future and is reflected in this weekend's Rome Declaration.

But in a demonstration of division within the union, the declaration has already been through several drafts, with countries objecting to its wording.

Several countries expressed concerns at wording in earlier drafts calling for the "multi-speed Europe" scenario.

Poland and some other Eastern European countries feared they would become second-class members behind wealthier Western countries.

Greece appeared to be objecting more on principle than on issues with the text; the Greek government attempting to gain leverage on upcoming debt talks. The country remains in a dire financial state with a further bailout due later this year.

But the elephant in the room will be the absence of Britain.

Of all the pressures the EU faces, the blow Brexit caused can't be over-exaggerated and the challenge of extracting the UK from the club is huge.

The leaders in Rome know that the motivations which caused Brexit are felt across the union.