Citing eyewitnesses, the London-based activists say a man in his 50s named Dhondup set fire to himself at around 9:30 a.m. Monday morning.
They have a photo that purports to show his burning body inside the monastery walls.
Dhondup died at the scene, Free Tibet said. Police reportedly attempted to remove his body, but monks refused to allow them.
According to the Tibet Post, Dhondup left behind a wife and one son.
More from GlobalPost: Tibetans in Turmoil, an in-depth series
By Free Tibet's count, he was the 55th Tibetan to self-immmolate since March 2011, but the first to do so at Labrang.
The 300-year-old monastery is an important center of Tibetan Buddhism and one of the largest and most influential outside Tibet, according to the Telegraph.
It is also known as a focal point for resistance against Chinese rule: anti-Chinese protests took place there in 2008, the Telegraph said, while Beijing's chosen Panchen Lama left just a few days into his residence at the monastery after getting a hostile reception.
Dhondup's reported death comes two days after another Tibetan set fire to himself outside a nearby monastery in Gansu province.
Lhamo Kyap, 27, ran in flames toward policemen, shouting slogans against Chinese rule and calling for the exiled Dalai Lama to return, Radio Free Asia reported. He later died of his burns.
There has been a spate of such protests this month, most of them fatal. At least four Tibetans had self-immolated by Oct. 13.
Meanwhile the prime minister of Tibet's government-in-exile, Lobsang Sangay, called on Beijing to enter into talks on greater independence for the region, a move that GlobalPost's Cain Nunns described as an "olive branch."
More from GlobalPost: Tibet calls for talks amid fiery protests