In the grainy security video, a man in a yellow shirt sits on a bench at the crowded Erawan Shrine, removes a backpack he is wearing, and leaves it behind when he walks away.

For police hunting who was responsible for Monday's deadly bombing in central Bangkok, there was no doubt about the man with youthful shaggy dark hair and glasses.

"The yellow shirt guy is not just the suspect. He is the bomber," police spokesman Lt. Gen. Prawut Thavornsiri told The Associated Press on Tuesday.

Prime Minister Prayuth Chan-ocha called the shrine bombing near a busy intersection that killed 20 and wounded more than 100 "the worst incident that has ever happened in Thailand," and he promised to track down those responsible.

"There have been minor bombs or just noise, but this time they aimed for innocent lives," Prayuth said. "They want to destroy our economy, our tourism."

Thai people light candles on Tuesday for the victims of an explosion that occurred Monday near the Erawan Shrine in Bangkok. (Sakchai Lalit/Associated Press)

Bangkok was rattled again Tuesday when another pipe bomb blew up at the Sathorn Pier, which is used by tourists, although no one was hurt.

Prawut released several photos of the man, with and without the backpack, on social media. The images were apparently taken from closed-circuit video at the shrine before the bomb exploded.

Video posted separately on Thai media appeared to show the same man sitting on a bench at the shrine, taking off the backpack and leaving it behind as he walked away.

Without elaborating, the prime minister said, "Today we have seen the closed-circuit footage, we saw some suspects, but it wasn't clear. We have to find them first."

Prayuth said the government will expedite "all investigative efforts to find the perpetrators and bring them to justice."

The bomb, which police say was made from a pipe and weighed 3 kilograms, went off about 7 p.m. in an upscale area filled with tourists, office workers and shoppers. No one has claimed responsibility.

Prawut said Tuesday's blast at the Sathorn Pier frequented by tourist boats also was caused by a pipe bomb and could be related to the shrine attack.

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Police said the bomb was thrown from the Taksin Bridge and fell into the Chao Phraya River, where it exploded. Security video showed a sudden blast of water over a walkway at the pier as bystanders ran for safety.

The Erawan Shrine is dedicated to the Hindu god Brahma, but is extremely popular among Thailand's Buddhists as well as Chinese tourists. Although Thailand is predominantly Buddhist, it has enormous Hindu influence on its religious practices and language.

Thai authorities identified five victims as Thai and four as Chinese — two of them from Hong Kong — along with two Malaysians and one Singaporean, and said the nationalities of the other eight victims remained unknown.

The British Foreign Office said one victim was a British citizen named Vivian Chan who lived in Hong Kong. It was not immediately clear whether she was one of the two Hong Kong victims identified by Thai officials. Officials at London's BPP University said she had studied there.

The blast occurred at a major Bangkok intersection near the Erawan shrine, a popular site for tourists. (Google Maps)

Defence Minister Prawit Wongsuwan said authorities had no idea an attack had been planned.

"We didn't know about this ahead of time. We had no intelligence on this attack," he said.

Prayuth vowed to "hurry and find the bombers," though he noted there may be just one perpetrator. Speaking to reporters, he continued what has been a notoriously prickly relationship with the media since the former general took control in a May 2014 coup that ousted a civilian government.

Asked if there were leads on the suspects, Prayuth bristled: "We are still investigating. The bomb has just exploded — why are you asking now? Do you understand the word investigation? It's not like they claim responsibility."

In Washington, State Department spokesman John Kirby said the U.S. is offering to support Thailand in its investigation but has not received a request. He said there was no indication that any Americans were among the casualties.

Thai Prime Minister Prayut Chan-ocha gestures as he speaks to journalists a day after the bomb attack, at Government House in Bangkok. (EPA)

Thailand has seen many violent attacks in recent years, particularly in a more-than-decade-long insurgency by Muslim separatists that has killed over 5,000 in the country's south. Those attacks have never reached the capital, however.

Bangkok has seen politically charged violence in the past decade; the deadliest, in 2010, killed more than 90 over two months and was centred on the same intersection where Monday's bomb went off. But none of those attacks included a bomb that seemed intended to produce mass casualties.

'Major ramifications'

Matthew Wheeler, Southeast Asia security analyst for the International Crisis Group, said the bombing was a "new type of attack for Bangkok" that doesn't bear the marks of typical violence in the past decade from political instability or separatists.

"It is certainly not like politically motivated attacks we've seen in the past which have generally been designed to grab attention but not cause casualties," Wheeler said, adding that he expected it would have "major ramifications for security in Thailand."

The suspect seen in the video wearing a yellow shirt raised initial questions about whether the violence was politically motivated, since one group of previous protesters was known to wear that colour of clothing. But officials have not linked the attack to Thai politics.

In the morning, investigators surveyed the damage as police and soldiers guarded the area, still littered with shattered glass and other debris. The normally busy intersection was closed to traffic and eerily empty, aside from onlookers standing behind police tape to take pictures. Barricades were set up outside nearby five-star hotels, and security personnel stopped cars to inspect trunks before letting them pass. The intersection was reopened by midday.

Workers clean a statue of Hindu god Brahma at the Erawan shrine on Tuesday, a day after a deadly blast near the shrine in central Bangkok. (Athit Perawongmetha/Reuters)

At least 20 people were confirmed dead and 126 injured, according to the Erawan Emergency Center.

Bangkok has been relatively peaceful since the military coup after several months of sometimes violent protests against the previous government.

At the same time, the military government has tightly controlled dissent, arresting hundreds of its opponents and banning protests. Tensions have risen in recent months, with the junta making clear it may not hold elections until 2017 and wants a constitution that will allow some type of emergency rule to take the place of an elected government.

Stirring the pot has been exiled former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup. His sister, Yingluck Shinawatra, was ousted as prime minister last year.

In a YouTube post last week, Thaksin urged his followers to reject the draft constitution because he said it was undemocratic. The draft charter is supposed to be voted on next month by a special National Reform Council. If it passes, it is supposed to go to a public referendum around January.