Thai authorities say the man who carried out a devastating Bangkok shrine bombing that killed 20 people was a foreigner and part of a network.

Police on Wednesday released a sketch of a young, bespectacled man shown on security footage walking into the crowded Hindu shrine and placing a backpack believed to contain the bomb.

A few hours later, a Bangkok court issued an arrest warrant for the man, describing him as a foreigner but not identifying him any further.

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The warrant, issued by Bangkok’s Southern Criminal Court, accused an “unnamed foreigner of premeditated murder, attempted murder and bomb-making”.

Before the arrest warrant was made public, national police chief General Somyot Poompanmoung told reporters a “network” had orchestrated the attack, which has no precedent in the Thai capital.

“We believe there must be people helping him, Thai people,” Somyot told reporters, referring to the prime suspect.

The attack occurred on Monday evening as worshippers and tourists crowded into the Erawan shrine in Bangkok’s commercial heart.

At least 13 foreigners from China, Hong Kong, Britain, Indonesia, Singapore and Malaysia were killed.

Another 68 people remained in hospital on Wednesday, 12 of whom were in critical conditions.

Buddhist monks chanted as they led the morning ceremony that drew together devotees. Photo: AAP

The police sketch of the suspected bomber showed him with black-rimmed glasses, a full head of dark hair and a light complexion.

Police spokesman Prawut Thavornsiri said his features were gleaned partly from security footage taken of him at the shrine in the minutes before the attack.

That footage, released to the public on Tuesday, showed him wearing a bright yellow T-shirt and dark shorts, walking into the shrine with a backpack.

He casually placed the backpack underneath a bench and then slowly walked away clutching a blue plastic bag while looking at what appeared to be a smartphone.

Police said he escaped on a motorcycle taxi, a common form of transport in Bangkok, and that the bomb exploded several minutes later.

Prawut, the police spokesman, said other security footage of the suspect was also used to create the sketch.

Police had initially said a second explosion at a Bangkok pier on Tuesday that caused no injuries may also be linked, deepening fears for residents of their safety.

On Wednesday, Somyot said the second attack might also be a “copycat” although police were keeping all options open.

A woman pays her respects at the Erawan shrine. Photo: AAP

Security experts had previously said they were baffled over who was responsible.

The arrest warrant announced on Wednesday immediately raised questions over which foreign groups had the motives and capabilities to carry out the attack.

The Islamic State group, which has caused carnage in the Middle East, is not known to have carried out any attacks in Southeast Asia.

Some Thai media outlets had cast suspicion on militants from China’s Uighur minority, a group that faces cultural and religious repression in their homeland.

The Erawan shrine is close to Bangkok’s Chinatown and Chinese tourists are among the most important for Thailand.

Five of the confirmed fatalities were from China, and another two were from Hong Kong.

But Uighur groups are not known to have ever carried out an attack outside of China.

The shrine, a popular tourist attraction that typifies the kingdom’s unusual blend of Hindu and Buddhist traditions, reopened on Wednesday morning with a ceremony led by chanting monks.

Its surroundings had been largely restored and the pools of congealed blood scrubbed away.

Twisted iron railings were the only immediate sign of the blast point, which police believe was caused by a bomb made up of three kilograms of high explosives.

CCTV images show a person suspected to be behind the Bangkok bombing.

-AAP