The grenade was found by the Air India crew on board the Boeing 747-400 which was operating the Mumbai-Hyderabad-Jeddah flight, airline sources said today.
On landing at Jeddah, the incident was reported to the local security agencies who took over the aircraft, the sources said.
The aircraft, which has been cleared and released by Jeddah airport security, is still in the city as Indian security personnel were investigating the matter. The plane is likely to return to Calicut soon.
The aircraft had been kept as a standby for the recent US visit of Modi. The Prime Minister had undertaken a five-day visit to the US from September 25.
"A suspicious object was found on board this aircraft. The investigations are on," an Air India Spokesperson told PTI.