By Francesco Guarascio
BRUSSELS, Dec 1 (Reuters) - European Union leaders on Sunday celebrated the 10th anniversary of the Lisbon Treaty, the union's legal cornerstone, amid calls to reform a bloc weakened over the past decade by economic and migration crises, rising euroscepticism and Brexit.
Sunday was also the first day at work for a new EU executive under its German president, Ursula von der Leyen.
"Europe is a promise, is future, is something we all have to build, brick by brick and day by day," von der Leyen said.
Ten years after the Lisbon Treaty came into force, a bloc conceived to forge unity from the ashes of World War Two finds itself beset by divisions.
Germany and France, long the EU's main axis, last week called for a "Conference on the Future of Europe", reporting by mid-2022, to make the EU "more united and sovereign". .
The Lisbon Treaty simplified decision making in a union recently expanded to 28 members with the inclusion of 12 states from the former Soviet bloc.
It strengthened the role of the elected European Parliament and reduced governments' veto powers over legislative changes, but still left several areas such as foreign policy and tax requiring unanimity - and thus vulnerable to veto from a single member state.
In 2015, the bloc embarrassingly failed to apply a common asylum policy to tackle an influx of refugees from the Middle East - its worst migration crisis since World War Two.
It is also divided about economic reform, raising fears that the euro currency, shared by 19 of its member states, may not be strong enough to withstand a future financial crisis. (Reporting by Francesco Guarascio @fraguarascio; Editing by Kevin Liffey)