Two Singaporeans injured in the blast that killed at least 20 people at Erawan Shrine in Bangkok have been transferred back home and taken to Singapore General Hospital for treatment.

Madam Michelle Heng, 58, and her 21-year-old son Donovan Chan were brought back to Singapore on Thursday to seek medical care and were admitted to SGH at 2.40pm for treatment.

The hospital declined to comment on the severity of their wounds, according to a ChannelNewsAsia report.

According to earlier reports, Mr Chan suffered burns to his right side and superficial cuts from tiny pieces of shrapnel in Monday's blast, and was unable to hear clearly in his right ear.

His mother had an operation on Tuesday morning to remove pieces of shrapnel in her right leg and was on a ventilator to help her breathe as she inhaled a lot of smoke.

When the incident happened, they were not with each other. Mr Chan was praying and his back was facing the area where the bomb went off.

He heard a loud blast and a lot of people fell down around him, but he did not fall.

Everything happened very fast, he previously told The Straits Times: "There were injured people everywhere and dead people around me."

He started looking for his mother, and found her sitting on the floor looking shocked. He tried to pull her up, but her leg was injured.

The blast was so strong that both Madam Heng's phones were damaged.

One Singaporean was among at least 20 people killed in the attack. At least seven other Singaporeans were injured.

No one has claimed responsibility for the attack.

Thai police on Thursday said they had questioned and freed one man who handed himself in after being seen on CCTV moments before the blast, but the prime suspect remains at large.