Maj Gen Khattiya Sawasdiphol, the controversial Army specialist, was pronounced dead at 9:20 am.
The director of the Vajira Hospital, Chaiwan Charoenchokthawwee, announced his death at a press conference.
Doctors tried but failed to revive him after his blood pressure dropped until his heart stopped. The drop in blood pressure caused him to develop kidney failure, prompting doctors to clean his kidneys.
His relatives will retrieve his body at 1 pm and funeral ceremony will be held at the Soamanus Temple.
Khattiya was shot in the head on Thursday evening while giving interview to foreign reporters at the Saladaeng Intersection.
His death was announced to the protesters at the Rajprasong by Jatuporn Promphan, Natthawut Saikua and Kwanchai Praiphana.
Several protesters broke into tear upon learning of the death.
They stood in silence for a minute to honour him.
Jatuporn said Khattiya's sacrifice of his life for the fight would lead to an uprising against the government.
He said Khattiya was a brother of red-shirt people who had shared suffering of the people.
"He has done his duty with bravery as best as he could," Jatuporn said.
Khattiya, who was better known as Seh [staff officer] Daeng, was an ardent opponent of the yellow-shirt movement and the current government.
His earlier warning that rallies of the yellow-shirt movement or the People's Alliance for Democracy, would be attacked by M79 attacks often turned too.
His earlier warnings that 'black-shirt warriors of unidentified forces' would fight against troops also turned too.
Earlier, he had conflicts with the red-shirt leaders after his plan to use the "people's army" was turned down by the red-shirt leaders who said they preferred peaceful fight.
But when the rally began on March 12, Khattiya stepped in to organise security affairs by the red-guards, claiming former prime minister Thaksin Shinawatra had assigned him to be in charge of security for the red-shirt demonstrators.