The 84-year-old grandmother of a Thai Olympic bronze medal winner collapsed and died on Monday while celebrating the weightlifter's success in Rio, police said.

The octogenarian was watching a live broadcast of her 20-year-old grandson, Sinphet Kruaithong, take third place in the 56-kilogramme division.

But a viral video showed the jubilant scene of cheering fans in his rural hometown transform into tragedy after his grandmother, Subin Khongthap, collapsed while watching the competition on a projector.

"The initial assumption is that she died from heart failure -- but we have to wait for the hospital's result," said Somwang Prangprakoan a police officer in northeastern Surin province.

"I'm not sure if she was too excited, or if maybe she was already ill," he added.

During an interview with local press ahead of the competition, Sinphet's grandmother said she missed her grandson but was overjoyed by his sporting journey.

"I cheer him on, fight fight! I miss my grandson and want him to be successful. He does this for the country to bring the gold medal back," she said.

Sinphet was the second Thai athlete to win a medal in Rio, following a gold secured by a female weightlifter in the 48-kilogramme category on Saturday.

Sopita Tanasan, 21, is the fourth Thai woman ever to win a weightlifting Olympic gold.