Part of the computer system of the Large Hadron Collider (LHC) was hacked into as the world's most powerful physics experiment got under way.
A group calling itself the "Greek Security Team" hacked into a computer connected to the system last Wednesday.
A spokesman for Cern, the lab that houses the LHC, said the hackers put up a message on the facility's website.
No harm was done but the incident has highlighted the need for security in the LHC's network, the spokesman said.
The hackers had targeted the computer network of the Compact Muon Solenoid Experiment (CMS), a huge detector that analyses data from the particle accelerator.
The LHC is attempting to recreate the conditions just after the Big Bang, in which the universe was created.
With the world watching as the first particles began circulating in the LHC, engineers were searching the hacked computer for possible malicious damage.
The CMS website displayed a page with a mocking message, in Greek, which included the line: "We are 2600 - don't mess with us".
As a result of the attack, the CMS webpage www.cmsmon.cern.ch, can no longer be viewed.
Cern spokesman James Gillies told the BBC that the compromised computer was not connected to the accelerator itself.
"The computer is used to monitor one of the experiments at the LHC, it's nothing to do with the LHC accelerator itself or any of the control systems," he said.
"It seems it was not a malicious hack and it was quickly detected and corrected but this sort of thing keeps you on your toes."
Mr Gillies said the LHC had a general access network and a more restricted access network which controls the sensitive systems.
He said that the experiment involved 10,000 scientists at 500 universities in 80 countries and that keeping on top of systems security was "not a trivial task".
"As far as I understand there was one user somewhere - who wasn't a hacker - who uploaded something on to this machine and inadvertently introduced a weakness that allowed people to get in," he said.
"Our IT department is constantly reminding the experimental collaborators of security issues regarding the network and will continue to do so," he said. "This may have strengthened their message."
The number 2600 is often used by the hacking community. It is believed to have originated in the US in the 1960s with the discovery that a tone of 2600Hz played down the line could be used to access restricted parts of the national telephone system.