Several small bombs exploded across Bangkok on Friday, rattling the Thai capital as it hosted a regional summit attended by US Secretary of State Mike Pompeo and leaving three people wounded.
Thailand, which has a grim history of political violence, remains deeply divided after a controversial March election returned a Thai junta to power as a civilian government.
Prime Minister Prayut Chan-O-Cha, who led the junta, was told of "the bombing incidents and has ordered an immediate investigation", Thai government spokeswoman Narumon Pinyosinwat said.
"Security measures have been tightened. The public is urged not to panic," she added.
Small devices — believed to be so-called "ping pong bombs" around the size of a table tennis ball — exploded at several locations across the city.
They appeared to be symbolic attacks aimed at embarrassing the government during the major summit but not designed to cause mass casualties.
"Three people received slight injuries from shrapnel," said Renu Suesattaya, director of Suanluang district where the first bombs were reported.
"I received a report that they are 'ping pong bombs' hidden in bushes by the road."
Two further explosions shattered glass near a well-known downtown skyscraper, emergency police added.
Bomb disposal experts were deployed around the Mahanakorn Tower -- owned by the King Power group that counts Leicester City football club among its assets.
At least three other blasts hit the area around a government complex, authorities said.
The bombings took place just before a keynote speech by Pompeo, who has joined the Southeast Asian Foreign Ministers meeting, in which he praised Thailand for rejoining the "democratic fold".
Thailand's government urged the media to avoid speculation on the motive for the bombings.
"We do not know yet how many people are involved," deputy prime minister Prawit Wongsuwon told reporters, adding that "people who do it want to stir the situation".
The blasts come weeks after former junta leader Prayut was inaugurated as a civilian prime minister, sparking outcry among many pro-democracy supporters in a kingdom scored by divisions.
The junta manoeuvred itself back into power with the help of a fully appointed senate stacked with army loyalists and an electoral system its critics say was designed to limit the success of the pro-democracy parties.
A slew of court cases since then targeting an emergent anti-military group popular have rankled government critics, especially younger voters.
The older "Red Shirt" pro-democracy group has also reacted with outrage to the election, but has so far remained off the streets with the army seemingly in an unassailable position.
Mass protests, coups and short-lived governments have defined Thailand's recent history, which is peppered with deadly bombings and shootings linked to politics.
Thailand's last hosting of the Association of Southeast Asian Nations (ASEAN) in 2009 was also overshadowed by unrest.
Then, protesters from the pro-democracy "red shirt" faction smashed their way into the summit venue in the resort city of Pattaya demanding elections.
Pandemonium ensued, with a number of leaders having to be rescued from a hotel roof by Thai army helicopters while others fled by boat.