By Renee Maltezou and Valentini Anagnostopoulou

ATHENS, Jan 4 (Reuters) - Flights across Greece were grounded for several hours on Sunday after a collapse of radio frequencies crippled air traffic communications, stranding thousands of travellers and bringing airport operations to a virtual standstill.

Authorities said the disruption began at 8:59 a.m. local time (0659 GMT), when most aviation radio frequencies were hit by massive interference, forcing a precautionary shutdown of Greek airspace.

Officials described the incident as unprecedented in scale, delaying dozens of flights during one of the busiest weekends of the holiday season.

Greece's civil aviation authority said an indeterminate "noise" impacted radio channels, but the cause was not clear.

"The 'noise' observed in the frequencies was in the form of continuous, involuntary emission," it said in a statement.

The disruption affected flights across the country for several hours, with authorities only able to service flyovers.

"For some reason all frequencies were suddenly lost .. We could not communicate with aircraft in the sky," Panagiotis Psarros, chair of the Association of Greek Air Traffic Controllers, told state broadcaster ERT.

He later told Reuters that the outage highlighted the vulnerability of an aging system, which he said should have been replaced many years ago.

"We work with the most antiquated systems...in Europe," he said.

By Sunday afternoon, limited services were restored after pilots switched to backup frequencies to keep in touch with controllers on the ground. Around 45 flights were leaving Greek airports every hour by late afternoon, an official said.

Christos Dimas, Greece's infrastructure and transport minister, said the incident did not compromise flight safety.

'UNPRECEDENTED' OUTAGE 

The air traffic controllers association said the breakdown affected all frequencies used on the ground, and some frequencies used by Athens Approach, an air traffic control unit responsible for managing aircraft flying in and out of Athens' Eleftherios Venizelos airport.

Among its responsibilities are radar monitoring for safe separation of aircraft in the sky as well as issuing instructions on speed, and altitude levels.

The association said controllers were using all means at their disposal to ensure the safety of flights, calling the scale of Sunday's incident "unprecedented and unacceptable".

Psarros said the problem seemed to be a collapse of central radio frequency systems at the Athens and Macedonia area control systems, the largest air control facility in the country. It monitors the Athens Flight Information Region, a vast expanse of airspace under the control of Greek authorities.

(Reporting by Renee Maltezou, Valentini Anagnostopoulou, Lefteris Papadimas and Angeliki Koutantou; Writing by Michele Kambas; Editing by Hugh Lawson and Ros Russell)