"The pilot has turned blue," a passenger said in a mobile text message to his cousin, Greek television reported. "Cousin farewell, we're freezing," it said.
Greek TV station Alpha reported that the pilot had told air traffic controllers the plane was experiencing air conditioning problems. Moments later, communications with the plane were cut.
Greek police and firefighters at the crash site said there were no immediate signs of survivors. Plane wreckage was scattered widely about the mountainous, uninhabited area, about 40km north of Athens.
"We have yet to locate any survivors. There is a small fire still burning, but it will be dealt with very quickly," a firefighter at the scene told Reuters.
Akrivos Tsolaki, head of the accident investigation committee, told reporters at the crash site the plane's two Black boxes - voice and data recorders - had been located.
Two Greek F-16 fighter jets were scrambled after the Helios Airways jet, en route from Larnaca in Cyprus to Prague via Athens, lost contact with the control tower at Athens international airport.
One of the F-16 pilots reported that he could not see the captain in the cockpit and his co-pilot appeared to be slumped in his seat, a Defence Ministry official told Reuters.
The aircraft crashed onto
mountains north of Athens
Greek police said there were no signs the plane had been hijacked.
Cypriot airport officials said flight HCY522 left Larnaca at 9am and lost contact at 10.30am.
The pilot appeared to have lost consciousness due to a loss in cabin pressure in the cockpit, Larnaca airport officials said on Cyprus state television CYBC.
"I saw the plane coming. I knew it was serious or that it was some kind of VIP because I saw the two fighter jets," said witness Dimitris Karezas, who owns a summer camp in the area.
"Two, three minutes later I heard a big bang," he said.
A Greek police spokesman said there had been 115 passengers and six crew members on board, of which 59 and eight children were heading to Athens, with 48 continuing on to Prague.
A spokeswoman for the Czech Airport Authority, Anna Kovarikova, said the flight had been due to land in Prague at 1.10am (1110 GMT).
Helios, Cyprus' first private
carrier, was established in 1999
As the extent of the disaster became clear, Greek Prime Minister Costas Karamanlis broke off his holiday on the Greek island of Tinos to rush back to Athens.
Cypriot President Tassos Papadopoulos was heading to Larnaca, where frantic relatives and friends began gathering outside the offices of Helios.
At the airport in Prague, where friends and relatives were gathering to meet the flight, screens showing departures and arrivals read simply "delayed".
Helios, Cyprus's first private carrier, established in 1999, flies to Dublin, Sofia, Warsaw, Prague, Strasbourg and several British airports using a fleet of Boeing B737 aircraft.