Army chief Gen Prayuth Chan-ocha regretted that the Victory Monument rally site was attacked with grenades, Deputy Army Spokesman Col Winthai Suwaree said.
He said Prayuth instructed Army officers concerned to coordinate with the Centre for the Administration of Peace and Order to adapt joint security checkpoints to prevent more attacks.
The spokesman said the Army officers would also coordinate with Police Commissioner-General Pol Gen Adul Saengsingkaew to have police speed up probe into all the attacks.
Winthai said police could not yet pinpoint the masterminds of the attacks.
He said the attacks might prompt more people to come out to join protests to demand justice and the situation could worsen.