The Large Hadron Collider – built since 2003 at a cost of USD 3.8 billion – provides scientists with much greater power than ever before to smash the components of atoms in a bid to see how they are made.

"The beam is the size of a human hair," Paola Catapano, a spokeswoman for the host European Organisation for Nuclear Research said after the protons were fired into the accelerator below the Swiss-French border at 0732 GMT.

The organisation, known by its French acronym CERN, is firing the protons – a type of subatomic particle – around the tunnel in stages, several kilometres at a time.

Once the beam has successfully been tested in clockwise direction, CERN will send it counter clockwise. Eventually the two beams will be fired in opposite directions with the aim of smashing together protons to see how they are made.

The start up – eagerly awaited by 9,000 physicists around the world who will conduct experiments at CERN – overcomes the objections of some sceptics who fear the collisions of protons could eventually imperil the earth.

The sceptics theorise that a by-product of the collisions could be micro black holes, subatomic versions of collapsed stars whose gravity is so strong they can suck in planets and other stars.

"It's nonsense," said James Gillies, chief spokesman for CERN, before the experiment’ start.

CERN is backed by leading scientists like Britain's Stephen Hawking in dismissing the fears and declaring the experiments to be absolutely safe.

Gillies said that the most dangerous thing that could happen would be if a beam at full power were to go out of control and that would only damage the accelerator itself and burrow into the rock around the tunnel.

And full power is probably a year away.

"On today we start small," said Gillies. "What we're putting in to start with is one single low intensity bunch at low energy and we thread that around. We get experience with low energy things and then we ramp up as we get to know the machine better."

He said a good result for today would be to have one beam going all the way around the tunnel in a counter clockwise direction. If that works, the scientists will then try to send a beam in the other direction.

"A really good result would be to have the other beam going around, too, because once you've got a beam around once in both directions you know that there is no show stopper," Gillies said. "It's going to work."

However, if there is some blockage in the machine, experts will have to go in and fix the problem, and that could take time.