He pointed out that closer cooperation with different speeds among individual member states did not represent a threat that European Union could be turned into a narrower group of countries that would make decisions for all.
On a warm, sunny Saturday morning, several leaders took to the stage in the same hall where the Treaty of Rome was signed to praise the EU's accomplishments - its creation after the Second World War ushered in the longest period of peace in the continent's history - while not denying its need for reinvention.
The lavish events to mark the 60th anniversary of the Treaty of Rome, the ancestor of today's European Union, emerged as a display of collective geo-political anxiety as much as a celebration of the region's remarkable accomplishments. The Prime Minister has now set out the Government's plan to achieve a new positive and constructive partnership between the United Kingdom and the European Union.
They also committed to boost EU's social dimension, and to make Europe stronger at global level through both existing partnerships and common security and defence.
"Daunting as they are, the challenges we face today are in no way comparable to those faced by the founding fathers", he said, recalling how the new Europe was built from the ashes of World War II. Let me be clear: as the Prime Minister has said, while we are leaving the European Union, we are not leaving Europe.
But the message from most of the protesters here in Rome is this: The EU is worth saving, but it has to better respond to Europeans' needs and desires.
The Rome Declaration, which was negotiated in deep secrecy by diplomats representing the countries' governments, shies away from acknowledging past mistakes or providing an explanation why the European Union got bogged down, and why populists in key countries are on the rise.
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Secretary of State Rex Tillerson, the former CEO of oil company Exxon Mobil, has recused himself from the decision-making process. Including work indirectly related to the construction, the number of jobs balloons to 42,100, the State Department estimated.
He argued that citizens may today not be experiencing the social advancement experienced in the past, leading to skepticism in the European Union project.
"We have to do better with Europe", says medical student Claudia Zorzi, 28, from the northern Italian city of Verona. Many people in Europe - and America - have doubts about the benefits of globalization. Britain's economy is one of the strongest in the world and we are well placed to face the future. The EU has also increasingly contributed to peace and progress in many other regions.
Montevecchi, an Italian-language tutor before joining the Five Stars, complains the European Union is not a gathering of people, but a bureaucratic structure whose main concerns reflect the interests of banks, lobbyists and multi-national companies.
The Polish PM had said earlier this week that she would not accept the declaration if it did not address issues that are a priority for Poland.
Pro- and anti-EU protests took place in Rome, while in London tens of thousands of people marched against Brexit, which May will trigger on Wednesday.
With British Prime Minister Theresa May absent, the other 27 countries signed a new declaration on the Capitoline Hill where six founding states signed the Treaty of Rome on March 25, 1957.
Montevecchi is part of a demographic that frustrates young pro-Europeans such as Simone Fissolo, head of Gioventù Federalista Europea - the Italian chapter of Young European Federalists.