The civilian was caught in the crossfire between insurgents and police, while the four officers died in a crash en route to provide backup to their colleagues battling the gunmen.
Pol Capt Prachaup Nimruang, head of a bomb disposal team in Narathiwat, said he and 15 other officers were returning to their base in four cars after inspecting a bombing incident in Rueso district on Friday afternoon.
The team decided to take a shortcut through the Yue La Pae mountain range, between Rue So and Yee Ngor districts. As they passed through, a bomb placed on a rocky slope went off and the rocks tumbled onto the road, he said.
At least 20 gunmen came out of hiding, fired at the four vehicles and threw nails on the road to prevent them from escaping. Three officers were injured in the shootout which lasted over an hour and the militants later fled.
During the gunfight, a civilian car passed by the area and was shot at. The driver of the vehicle was injured while the person sitting next to him was killed.
In Bajoh district, a police car overturned after its front tyre burst, resulting in four deaths and seven injuries.
The vehicle was carrying officers who were heading to the Yue La Pae range to provide backup for the ambushed bomb disposal squad.
The deceased officers were Pol Lt Arkom Intornsomboon, Pol Lt Noppanai Kiritarn, Pol Lt Apichai Joonruang and Pol Lt Wuthipong Pooparn.
Another team of officers from the Yee Ngor police station were also travelling to Yue La Pae, but they encountered a roadside bomb and spike belts along the way. Nobody was hurt, however.
The obstacles in the road caused the reinforcements to arrive at the mountain range 30 minutes later than planned.
The afternoon incidents followed the drive-by shooting of a woman in Rangae district earlier in the day.
Pol Lt Apichart Dorloh, a duty officer at the Rangae police station, said the woman was identified as Jareewan Srisamor, a resident of Moo 3, tambon Kalisa.
According to witnesses, Ms Jareewan was travelling on her motorcycle along a local road when a gunman on another motorcycle fired at her with a handgun. The assailant then fled.
The woman took six bullets to the body and was taken to Narathiwat Ratchanakarin Hospital.