Two people were slightly injured when the People's Democratic Reform Committee (PDRC) anti-government rally site in Lumpini was attacked for three hours shortly after midnight.
According to reports, about 20 grenades were used to attack the area.
The first grenade came shortly after midnight close to Gate 4 of Lumpini Park near the Thai-Belgian flyover.
Gunshots followed in the area, then near Sala Daeng and Surawong Road, and Henri Dunant intersection.
Between 1.55am and 4am, more than 18 blasts were heard near Henri Dunant and Sarasin intersections while gunshots continued to bombard the Thai-Belgian flyover. However, the attackers were unable to enter the area around the protest-site stage.
Forensic officers examine the damage from gunfire at Homa Travel and Tour on Silom Road. (Photo by Thiti Wannamontha)
Vendors on Thaniya and Silom roads said they heard the blasts for almost two hours from before 2am. None of them were injured.
At 3am, PDRC guards extended their road closures to Thaniya intersection and searched the area for suspects. Reporters and police were not allowed to enter.
There were reports that the guards caught a suspect, a taxi driver who tried to flee on Rama IV Road, heading for Hua Lumphong train station.
Suchart Srisung posted on his Facebook page to say that very loud explosions had been heard near Gate 5 of Lumpini Park at 4.11am, forcing a shutdown of the park.
A Lumpini station police officer confirmed three guards were also shot at Gate 4.
In light of the escalating political violence, the army has adjusted its security measures and the deployment of civic-action troops to improve efficiency in monitoring "risk spots", Col Winthai Suwari said on Tuesday.
The army's spokesman said more military checkpoints will be set up to cover risk spots around PDRC rally sites, and at the offices of independent agencies, the judiciary and other locations deemed prone to violent attacks.
Routes for military patrols will also be reviewed and patrols will be more frequent.
The army units assigned to civic action will continue to perform their duties, serving both the authorities and the people, the spokesman said.
Col Winthai said the adjustments were agreed at a meeting chaired by Gen Prayuth Chan-ocha, the army chief, at the First Infantry Division headquarters.
According to the Bangkok Metropolitan Administration Medical Department's Erawan Centre, 20 people have been killed and 720 wounded in political violence since Nov 30 last year.
Of the wounded, 33 are still hospitalised.