Images show the man carrying a rucksack near the Erawan Shrine, a major tourist attraction, in the popular Ratchaprasong district then leaving the area without it around 15 minutes before the blast.
Thai Prime Minister, Prayuth Chan-O-Cha, said the suspect, who was wearing a yellow T-shirt and thick-rimmed glasses, was believed to be from an “anti-government group based in Thailand’s north-east” – the heartland of the kingdom’s anti-coup Red Shirt movement.
He described the bombing as the “worst ever attack” on his country.
“This is the worst incident that has ever happened in Thailand,” Prayuth said.
“There have been minor bombs or just noise, but this time they aim for innocent lives. They want to destroy our economy, our tourism.”
Members of the public are also handing in their own video coverage.
It is believed the bomb was placed to cause maximum casualties, and there is a lingering sense of shock in Thailand, which until now had been spared the kinds of terrorist incidents experienced by some of its neighbours.
Most victims were Thai, but at least eight were foreign – mostly from elsewhere in Asia.
In currency trading, the Thai baht fell to its lowest level in six years.
National police chief, Somyot Poompummuang, earlier described the device as a pipe bomb – an improvised device where explosive material is put in a sealed cavity to maximise the explosive impact.
He said the 3kg bomb had been placed inside the shrine.
“Whoever planted this bomb is cruel and aimed to kill. Planting a bomb there means they want to see a lot of people dead,” he said.
Marko Cunningham, a New Zealand paramedic working with a Bangkok ambulance service, said the scene was “like a meat market”.
“There were bodies everywhere. Some were shredded. It was horrific,” he told the Reuters news agency.
Bangkok is on edge as another bomb exploded near a rail station but there have been no injuries.
“No one was killed or injured. Police are at the scene to investigate what kind of device it was,” an officer at Yanawa police station, who asked not to be named, told AFP.
He said the incident took place shortly after 1pm (0600 GMT) near the Saphan Taksin BTS skytrain station.