A man suspected of planting a deadly bomb in Bangkok is part of a wider "network", Thailand's top policeman said Wednesday, as monks led prayers and reopened the shrine where 20 people were killed.

The blast struck on Monday evening as worshippers and tourists crowded into the Erawan shrine in the Thai capital's commercial heart, but with no claim of responsibility the motive and identity of the culprit remain a mystery.

The unprecedented attack left at least 11 foreigners dead, with visitors from Britain, China, Hong Kong, Indonesia, Singapore, and a family from Malaysia among the victims.

Another 68 people remain in critical condition after a blast that shredded bodies and incinerated motorcycles at one of the city's busiest intersections, sending a shockwave through the country's pivotal tourist industry.

"It's a network," police chief General Somyot Poompanmoung told reporters without elaborating, a day after CCTV footage emerged showing a suspect leaving a backpack moments before the blast struck.

"We believe there must be people helping him, Thai people," he added, appearing to rule out the action of a lone wolf attacker.

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 as police conceded they do not know who was responsible.

But on Wednesday Somyot said the second attack might also be a "copycat" and that police were keeping all options open.

Thai police have now offered a one million baht ($28,000) reward for information that leads to the arrest of the main suspect.

Widely circulated video of the suspect, apparently young and slightly built, and sporting glasses and shaggy dark hair, has prompted social media chatter that he could be a foreigner.

In a televised address Wednesday, Thai junta spokesman Colonel Winthai Suvaree said security has been tightened in tourist areas "especially where there are many Chinese tourists, to regain their trust and confidence".

The Erawan shrine -- a popular tourist attraction that typifies the kingdom's unusual blend of Hindu and Buddhist traditions -- and its surroundings have already 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.

Buddhist monks chanted as they led the morning ceremony that drew together devotees, including tourists, who knelt and held joss sticks.

A relative of the dead Malaysians had laid bundles of clothes and ceremonial money at the shrine to help their departed loved ones find comfort in the afterlife, witnesses said.

One devotee at the shrine had more reason than most to give thanks.

Tommy Goh, 56, a Thai-Malaysian from Penang, said only a delayed taxi from his hotel spared him from being at the shrine around the time of the blast.

"We were meant to be here around 6:50-7pm but the taxi didn't arrive from the hotel... so we went somewhere else," he told AFP.

"Ten minutes later and it could have been so different."


Bombs linked?

Police released images Tuesday showing the suspect, wearing a bright yellow T-shirt and dark shorts, walking into the shrine with a backpack.

In the security footage he casually places the backpack underneath a bench and then walks away clutching a blue plastic bag and what looks like a smartphone.

The bomb exploded several minutes later, leading Thai police to make the man their prime suspect.

There have been mixed messages from senior police about whether the first bomb and the small explosion on Tuesday are linked.

Colonel Kamthorn Ouicharoen, head of the police bomb squad, confirmed the bridge device -- which was hurled into the water and shocked pedestrians -- was the same type as the one detonated at the Erawan shrine.

"It's exactly the same, the equipment used to make it, the bomb size," he said.

Somyot had also previously suggested a link.

But a senior source at the National Security Council said the difference in execution and intent of the two attacks reduced the likelihood of a tie.

"The second bomb appears aimed at inciting unrest," rather than killing, the source told AFP, requesting anonymity.

Thailand has experienced a near-decade-long political crisis that has seen endless rounds of street violence.

But it has never seen anything on the scale of Monday's bomb and foreigners are generally not targets in a country that prides itself on extending a warm welcome to outsiders despite any political troubles.

A festering insurgency by Muslim rebels in the Thai south has claimed 6,400 victims -- the majority civilians -- but is a highly localised conflict.

Monday's attack has caught Thailand's junta flat-footed, raising security fears at a time of deepening discontent with the economy, a contentious new constitution and the kingdom's poor human rights environment.

The military seized power last May, casting its coup as a necessary evil to prevent the country imploding after months of protests against the former government.

But with the bomber still on the run ordinary Thais said they fear another attack.

"I'm still scared because you never know where they (the perpetrators) will strike again," 43-year-old Sommai Gazem told AFP.