Flight 9W 339, with 115 passengers and seven crew members, landed without incident at Ahmedabad and all 122 safely "deplaned", a Jet Airways spokesperson said.
A member of the cabin crew had found a "printed note" stating that there was a bomb in the belly, the cargo area, of the plane, a senior official of the Bureau of Civil Aviation Security (BCAS) told PTI.
The message was passed on to the pilot, who is believed to have pressed the hijack alert button, following which the plane made an emergency landing, he said.
Describing it as a bomb threat, he said nothing was found following a check of all the passengers and their bags.
PTI correspondent Rajkumar Leishemba, who was on board, said the flight was diverted to Ahmedabad citing "security reasons".
All the passengers deplaned and screened. They were photographed and questioned by security personnel on various details, including their last overseas visit, he said.
After more than six hours at the airport here, the flight carrying the passengers took off for Delhi around 10.30 am, he said.
"Passengers were profiled, taken photographs and all personal details sought...," Leishemba said on Twitter.
The B 737-900 aircraft was parked at a remote bay, the spokesperson said in a statement.
"Jet Airways is extending full cooperation to the security agencies who are investigating the matter and is not in a position to comment further at this stage," it added.
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