Thai Prime Minister Prayuth Chan-ocha ordered security to be tightened in Bangkok on Monday after two small bombs rattled a luxury shopping mall and stoked tension in a city under martial law since a coup in May.
Two people were slightly hurt but the blasts caused little damage on Sunday evening. They were the first to shake the capital since the military seized power to end months of sometimes deadly street protests.
"I have ordered security to be tightened because this case involves the well-being of the people," Prayuth told reporters.
"This case shows that we still need martial law ... there are still bad people disrupting the peace. We must find ways to severely punish them."
There was no claim of responsibility.
CCTV footage showed two possible suspects near where the pipe bombs exploded in the heart of one of Bangkok's busiest shopping districts but the images were unclear and they had not been identified, police said.
Defence Minister Prawit Wongsuwan said such violence "inflicts loss of confidence" in the country. Tourism accounts for about 10 percent of Thailand's economy.
Political tension has been high since last month when a national assembly hand-picked by the junta banned former Prime Minister Yingluck Shinawatra from politics for five years.
The decision angered supporters of Yingluck and her self-exiled brother, Thaksin Shinawatra, although there has been little sign of a return to the protests that have dogged Thailand for years. The military has been tough on dissent since the coup.
Thailand has weathered turbulent politics for a decade as former telecommunications tycoon Thaksin and his allies have vied for power with the Bangkok-based royalist-military establishment that sees the Shinawatras as a threat and reviles their populist policies.
The bombs were just a few hundred metres away from the site of a military crackdown on "red shirt" supporters of the Shinawatras in 2010.
Occasional crude bombs similar to those used on Sunday kept the capital on edge for months after the 2010 crackdown.
The motive for the Sunday blasts appeared to be to create panic, said junta spokesman Winthai Suvaree. The bombs were behind power transformers on a walkway linking an overhead rail line to the Siam Paragon mall, police said.
Sporadic violence during the six months of protests that preceded the May 22 coup claimed almost 30 lives.
Thailand's Muslim-dominated south has for years been racked by insurgency that has claimed thousands of lives. There was no indication the Sunday explosions were linked to the south.