Foreign ministers from the US, China and Russia are joining Asian counterparts for a security meeting in the city.
Six bomb blasts have wounded at least four people in Bangkok as Thailand hosts US secretary of state Mike Pompeo at a high-profile summit.
Explosive devices were found or detonated at various locations, including two at stations of the city’s elevated Skytrain rail system.
The first explosions were heard just before 9am local time on Friday near the centre of the Thai capital.
Three of the injured were women who were cleaning the street, and a fourth person was hurt after a device went off at one of Bangkok’s tallest buildings – the 77-storey King Power Mahanakhon building.
Erawan Medical Centre said none of victims were seriously hurt.
Six devices in total were detonated and another was recovered before it went off, said Colonel Kamtorn Uicharoen from Bangkok’s police force.
Government spokesman Narumon Pinyosinwat said Prime Minister Prayuth Chan-ocha had ordered an investigation.
“The situation is being closely monitored and security measures have been tightened. The public is urged not to panic,” he said.
The PM tweeted: “I would like to condemn those causing the situation which destroys peace and damages the country’s image.”
The blasts come less than two weeks after his former military junta transformed into a civilian government.
The country’s deputy PM, Prawit Wongsuwan, said the perpetrators were trying to create a “situation” and that two suspects had been arrested.
Foreign ministers from the US, China and Russia had joined their counterparts from southeast Asian countries for a security meeting in the city.
None of the blasts disrupted the proceedings which were already under tight security.
However, on Thursday, police found two fake bombs near their headquarters, close to the summit venue and arrested two people.
Small, generally harmless bombs, have been used in Thailand before.
Government opponents are often blamed but there is also speculation such incidents are part of a power struggle among Thailand’s politicised security forces.
Bangkok was last hit by small blasts in 2017, with 21 people wounded at a hospital.
An opponent of the country’s military regime – which ruled the country after the 2014 coup – was jailed over the attack.