Candle-bearing mourners gathered at a Bangkok shrine Monday a week after a bomb killed 20 people there, as Thailand's police chief said the investigation has been hampered by broken security cameras and other factors.

The bombing at the Erawan shrine in the capital's commercial heart was Thailand's worst single mass-casualty attack, but the trail is growing increasingly cold.

The main suspect is an unidentified man in a yellow T-shirt seen on CCTV placing a rucksack under a bench at the shrine minutes before the blast. He is still on the run, with police publicly no nearer to naming him or his motivation.

Late Monday monks dressed in saffron robes led prayers at the small streetside shrine.

Around 100 mourners -- some carrying lit candles in paper cups -- observed a minute's silence at 6.55pm -- the exact time the bomb went off on August 17 and caused carnage.

Many of the dead were Asian tourists. Scores of other people were wounded.

"I feel sad, the sadness has stayed with me," Nopparat Jantawisut, 68, told AFP.

"The people who died here came with good hearts to make merit but the result was the opposite."

The city remains jittery, with the bomber on the run and rumours and even bomb hoaxes abounding.

Earlier national police chief Somyot Poompanmoung said his officers were working at "full capacity with no rest".

But he added that the investigation has been hampered by many issues such as the fact that the majority of the city's security cameras were not working.

"Sometimes there might be 20 CCTV cameras on a road but only five of them work," he told reporters. "Another 15 might be broken for whatever reason."

Police also say they lack modern facial recognition technology to decipher who the bomber is from the grainy security camera footage.


'Police under pressure'

Somyot is under intense pressure from both the public and the ruling junta to catch the perpetrators of an attack that sent shockwaves through the vital tourist sector.

Although residents have tried to get on with their lives, tensions remain high.

Police are convinced the main suspect, who has been named in an arrest warrant as foreign, must have had help from a network, likely involving some Thais.

But Somyot said he was no nearer knowing whether he had fled the country or not.

"I say that I believe he is still in Thailand because I don't have information to confirm otherwise," he said.

Security footage has played a role in tracking some of the key suspect's movements.

Cameras have picked him up arriving at the shrine via a three-wheeled "tuk-tuk" taxi, leaving the bag and then departing on a motorbike taxi south towards the Silom area of Bangkok.

After that he disappears from view.

Police have interviewed the motorbike drivers, with Somyot adding that DNA tests had been carried out on 20 baht notes that the suspect may have used.

The police chief said he was also hoping for technical help from other countries which might have computer equipment that can quickly search large amounts of CCTV footage, as well as "make the picture more clear".

He said a number of countries had offered help but some of their equipment did not work with the Thai systems. Other nations had been sent evidence but had yet to get back with their findings.

Last week junta chief Prayut Chan-O-Cha publicly bristled at any suggestions Thailand might ask for foreign investigators, though he has said he is open to technical advice from overseas.

A sketch of the main suspect has been widely circulated and bounties totalling more than $300,000 have been offered for his arrest, prompting a flood of enquiries and amateur sleuthing.

But Somyot said "90 percent" of calls from the public turned out to be false leads.