Scientists and heads of state from around the world have gathered to mark 50 years of the European Organization for Nuclear Research, known as Cern.
French President Jacques Chirac was among those at the celebrations
French President Jacques Chirac and King Juan Carlos of Spain are among those attending celebrations at Cern's headquarters in Geneva.
Cern is the world's largest particle physics laboratory, whose research is conducted only for peaceful purposes.
It was formed in the aftermath of World War II to unite Europe's scientists.
Europe also wanted to stop a brain-drain to the United States.
King Juan Carlos said it was a centre of excellence which "attracted world experts in the field", the Associated Press news agency reported.
Birth of Universe
Over five decades, the centre has pioneered research into the sub-atomic world, deepening our understanding of materials that build the Universe and the forces that hold them together.
The tools its physicists developed to conduct their research have also been highly successful.
The world wide web started at Cern as a method of file-sharing.
But it is Cern's latest project which may prove to be its most significant, says the BBC's Imogen Foulkes in Geneva.
The Large Hadron Collider (LHC) will smash particles at near light-speed, recreating conditions that existed at the very birth of the Universe.
"With our particle colliders today, we're actually able to produce temperatures that are a billion times the temperature in the centre of the Sun," says Cern physicist Richard Jacobsson.
"This temperature allows us to study the Universe as it looked when the Universe was no older than a hundredth of a billionth of a second," he says.
And for those who are doubtful about the usefulness of such discoveries, Cern physicists point to the many achievements that have come about precisely because people have pursued fundamental questions, says our correspondent.