The son of ousted Thai premier Thaksin Shinawatra has more than doubled the reward offered by police for the capture of those behind this week's deadly Bangkok bomb blast by putting up nearly $200,000 (RM23,423).

Monday's attack on a religious shrine in a bustling Bangkok shopping district killed 20 people, mostly ethnic Chinese tourists from across Asia, leaving police scrambling to find the assailants and sending shock waves through the nation's vital tourism sector.

Police are convinced the attack was planned and coordinated by a network.

With no arrests and few clues on the identity of the main suspect in a yellow T-shirt seen leaving a backpack at the shrine moments before the fatal blast, police have offered a three million baht ($84,000) reward for any information that leads to the capture of the assailant.

That offer has now been more than doubled by Panthongtae Shinawatra, the first born son of populist leader Thaksin Shinawatra. Thaksin currently lives abroad in self-imposed exile after a corruption conviction that he insists was political.

"I have been given permission from my father to give seven million baht -- two for any informant and five million for those officials who investigate and make arrests," he wrote in a Facebook post late Friday.

"In order to swiftly regain confidence and morale among both Thais and foreigners we have to arrest the suspect as soon as possible to make everyone realise that Thailand is not (a) place where this kind of thing can happen and you get off scot free," he added.

Earlier in the week a prominent member of the Red Shirt movement loyal to Thaksin also offered a further two million baht reward -- bringing the total money on offer to $335,000.

Perpetrators still unknown

With no-one claiming responsibility for the bombing, rumours and speculation have swirled in Thailand over the country's worst single mass casualty attack in living memory.

Among the potential perpetrators named by police and experts alike include international militants, members of Thailand's southern Islamist insurgency and militants on both sides of Thailand's festering political divide.

Bangkok has endured a decade of deadly political violence amid a bitter power struggle.

On the one side stands the military, backed by the middle class and elite. On the other, the rural and urban poor loyal to Thaksin, toppled in a 2006 coup, and his sister Yingluck who was forced out of office days before the current junta seized power last year.

The ongoing political struggle has seen repeated rounds of deadly street protests, including grenade and small pipe bomb attacks.

But experts say neither side had much to gain by launching an attack of Monday's scale, risking opprobrium from both the Thai public and international community.

Thaksin himself has been vocal in voicing outrage at the attack.

"I condemn the perpetrator and anyone behind (this attack) with the strongest words," he wrote on his Twitter account earlier this week.

Thai police insist their investigation is making progress despite days of confusing and sometimes contradictory statements from senior officers and junta officials.

An arrest warrant has been issued for the main suspect, who is described as an unnamed foreign man with shaggy hair and thick rimmed glasses.

It is not known if the suspect has already fled the country, while authorities have also openly questioned whether he could be half-Thai or a Thai wearing a disguise.