Hailed as the foundation for a more democratic, efficient and transparent union, the treaty brings to fruition a long quest to rewrite the EU's internal rules - all the more pressing after 10 more countries joined in 2004.

"The treaty of Lisbon puts citizens at the centre of the European project," president Barroso said. "I'm delighted that we now have the right institutions to act and a period of stability."

The milestone was marked by a ceremony in Lisbon, where the treaty was signed two years ago.

For the charter to become a reality, all EU countries had to ratify it, a process only recently completed. In October, Irish voters returned to the ballot box - and this time endorsed the treaty. The Czech president subsequently dropped his opposition, removing the final hurdle.

Gearing up for the big day, EU leaders met late last month to fill two posts created by the treaty. They appointed Belgian prime minister Herman Van Rompuy as the EU's first full-time president, and commissioner Catherine Ashton as foreign policy chief.

For Mr Barroso, the treaty's debut means he can get on with the job of putting together a new commission, having announced the nominees last week. Likewise, 18 MEPs elected in June under Lisbon treaty rules can now claim their seats in the European parliament.

That's merely the start.

Among many other changes, the treaty redistributes voting weights between member countries, removing national vetoes in a number of areas. It expands the commission's powers and greatly increases parliament's involvement in the legislative process.

A new petition process will give citizens the opportunity to directly influence EU policy. The human rights charter becomes legally binding.

Lisbon amends the Rome and Maastricht treaties, giving the EU a new legal framework and tools to tackle challenges in an increasingly interlinked world.