The world's biggest atom-smasher, shut down after its inauguration in September 2008 amid technical faults, has restarted, a spokesman for the European Organisation for Nuclear Research says.
"The first tests of injecting sub-atomic particles began around 1600 (0200 AEDT Saturday)," CERN spokesman James Gillies told AFP on Friday.
He said the injections lasted a fraction of a second, enough for "a half or even a complete circuit" of the Large Hadron Collider built in a 27km-long tunnel straddling the Franco-Swiss border near Geneva.
"If all goes well tonight we will try to circulate a beam of particles for several minutes around 0700 (1700 AEDT)" on Saturday, Gillies said, adding that he would open a bottle of champagne if the accelerator kept on working.
The LHC promises to unlock scientific mysteries about the creation of the Universe and the fundamental nature of matter.
But the machine was shut down just nine days after its inauguration following a series of technical faults.
Since then, the LHC's components have been tested to an energy equivalent of five teraelectronvolts at full power.
The maximum output of what is currently the largest functioning collider in the world, at the Fermilab near Chicago in the United States, is one teraelectronvolt.
CERN had said in August that upon its relaunch, the LHC will run at 3.5 teraelectronvolts in order to allow its operators to gain experience of running the machine.
The first data should be collected a few weeks after the first particle beam is fired.
CERN said the partial power level will be kept until "a significant data sample has been gathered" and ramped up thereafter.
Designed to shed light on the origins of the universe, the LHC at CERN took nearly 20 years to complete and cost 3.9 billion euros ($A6.33 billion) to build.
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