Nassir Yima, security chief for the Network of Students and People for Reform of Thailand (NSPRT), said the incident took place at about 3.30am when a golden bronze Toyota car arrived at the site with an unknown number of men on board.
They rolled down a window in the back seat and one of them opened fire with an M16 rifle at the uards on duty behind a stage.
While the car was moving along Rama V road in the direction of the Wat Benjamabopit intersection and the Missakawan intersection, the gunman continued to spray bullets at the guards.
The gumen fled after the shooting.
The attack wounded four guards, two of them seriously.
Yutthana Ong-art, about 30, was admitted to Klang Hospital and later pronounced dead.
"The dead man is in his 30s. He was shot in the torso," a spokesman for the city's Erawan emergency centre said.
Hospital officials identified the three other guards as Prawit Thongpraeng, Surapong Somklaew and Saneh Lohasart.
Police recovered shells and other ammunition at the scene. They have yet to examine all the ammunition and declined to comment on the protesters' claim that the shooter had used an M16 rifle.
PDRC co-leader Sathit Wongnongtoey said that Saturday's drive-by shooting at Chamai Maruchet bridge showed there were certain people who wanted to incite unrest to discredit a protest that has been carried out peacefully without any weapons.
"The guards will now oversee the protest venue more rigorously to ensure the safety of people who come to join the demonstration," said Mr Sathit, a former Democrat Party MP. "I want to ask the police around the protest venue, why can't they ever prevent violence from happening?"
The incident on Saturday was the third death of the week. A police officer and a protester died of gunshot wounds suffered in clashes on Thursday at the Thai-Japanese sports stadium where political party registrations were being held.
In the Thursday incident, protesters were seen hurling bricks at police, who responded with rubber bullets and tear gas. It is not clear who fired the live ammunition, though both sides accuse a "third hand" of attempting to stoke further unrest.