A British tourist who was in a hotel directly above the bomb blast that has killed at least 18 people has described the chilling moment of detonation.

Former Bangkok resident Lucinda-Jane Chastain told Sky News: "It was just this massive explosive noise. The whole building shook.

"We all ran to the windows. It was quite hard to see what was happening but we could see debris in the street.

"All we can see is a horrible mess on the road. This is at the very heart of Bangkok.

"It's the last place in the world you'd expect something like this to happen."

The explosion happened outside the Erawan Hindu shrine in the downtown Chidlom district of the Thai capital.

Others witnesses described body parts strewn across the street and the dead covered in white sheets.

Thomas O'Malley, a Bangkok resident who was jogging near the scene of the explosion, said: "I saw smoke, fire, absolute chaos, and a fireball.

"I'll never forget what I saw. There were body parts in the street.

"The shrine is just off the main road and directly across are the two large shopping centres. It's a massive intersection, full of shoppers, so it's a dreadful place for this to happen."

Four foreigners are among the dead, according to media in Bangkok.

Some 117 people have been wounded, and local media claim most of them are from China and Taiwan.

"There were bodies everywhere. Some were shredded," said Marko Cunningham, a New Zealand paramedic working with a Bangkok ambulance service.

Mr Cunningham said the blast had left a two-metre-wide crater.

Local journalist Martin Lowe said: "It's a fairly chaotic scene. Police, fire and army officers are arriving here.

"The blast appears to have taken place right in the centre of the main shopping district. It's an area that's always frequented by tourists. Shops were open at the time of the explosion.

"What I can see now is roads being closed off and people being pushed away.

"I have seen a body covered with a white sheet by the side of the road and it looks like there are many more."

Mr Lowe added that police were slow to disperse people from the scene.

:: Analysis: Thai Bombs Try To 'Bend Country Out Of Shape'