Salem el-Ameel, a spokesman for a local force, said a resident reported the grave about a month ago on his farm in al-Daheir district, east of Sirte.
"At the time we did not have the capabilities to go and dig it up, but now we have dug up more than 75 bodies," he said. He says those buried at the site appear to have been wearing the clothes of IS fighters, but that forensics testing is needed to determine their identities and nationalities.
IS and other extremist groups exploited the chaos in Libya following the 2011 uprising that toppled and killed Moammar Gadhafi. Islamic State militants carved out a fiefdom centered on Sirte before Libyan militias drove them out 2016. The country is currently governed by rival authorities in the east and west, each backed by various militias.
In a separate development, the self-styled Libyan National Army said it seized two Italian fishing vessels off Libya's coast.
LNA spokesman Ahmed al-Mosmari told a news conference in the eastern city of Benghazi that the vessels were seized in Libyan territorial waters and brought to the port of Ras al-Hilal.
Italy's ANSA news agency reported that Libyan forces fired on the boats, causing damage to the cabin of the 140-ton Afrodite Pesca. The other boat, Mcv Pesca, sustained no damage, and no injuries were reported, ANSA said.
It quoted Nicola Cristaldi, the mayor of the town of Mazara del Vallo, where the boats are based, as saying that the seizure happened in waters that the Libyans "unilaterally and illegitimately" consider their own.
Italy plans to host talks among Libya's political factions in November. Italy's Deputy Foreign Minister Emanuela Del Re visited eastern Libya on Wednesday, meeting with the speaker of the Libyan House of Representatives in Tobruk, Aguila Saleh, and Field Marshal Khalifa Hifter, who leads the Libyan National Army.
He had concluded his visit before the seizure of the fishing boats was announced.
Libya is a former colony of Italy, which is just across the Mediterranean and has tried to work with Libyan leaders to restore stability and stem the flow of migrants through the North African country.