The blast hit in the evening peak hour when the shrine was packed with worshippers and tourists.
More than 120 people have been wounded.
Foreign nationals are among the casualties, and the Department of Foreign Affairs is working to find out if any Australians were affected.
No-one has yet claimed they carried out the attack.
There was panic in one of Bangkok's most popular districts.
A bomb went off near a popular Hindu religious site: the Erawan Shrine at Rajprasong intersection, an imporant tourist attraction in Bangkok's main shopping area.
It exploded during the evening rush hour.
This man works in an office close to the shrine and was on his way home when the blast occurred.
"I was just passing this Erawan shrine and I saw some Chinese tourists standing there and taking pictures, so I passed the intersection, walked past it and literally 30, 50 metres after I passed this site, the bomb went off and I got knocked over by the shock wave and I saw a flash of light and I looked over the railing and I saw the motorbikes knocked down on the street. I saw fire, I saw people bleeding on the street and screaming and people were running in all directions. It was pure mayhem."
The windows on nearby buildings were shattered; bodies and body parts strewn across the road.
Authorities say a citizen of the Philippines and a Chinese person were among the dead, without specifying a nationality.
Local taxi driver Sithit Manukham rushed to the scene after he heard the blast go off and tried to help the injured.
"I heard an explosion. My heart was beating fast but I wasn't afraid, so I came in to see. I saw wounded people with arms ripped off from the shoulder, another's leg was ripped off. I helped carry them into the hospital."
Newlyweds Holger and Nina Siegle, from Stuttgart in Germany, didn't see the blast, but were alerted by a friend's text
"When we left Germany, we did not think anything like this could happen here in Bangkok, like in the city centre. We were thinking about not going to Egypt, not going to Morocco, Tunisia, or Syria or whatever, but we were not thinking about not going to Bangkok. Our honeymoon and our vacation will go on, but with a very unsafe feeling."
Foreign Affairs officials have updated the smartraveller.gov.au website, asking Australians to keep away from the area and follow the advice of local authorities.
The level of advice has not changed.
The Department of Foreign Affairs is working with its Thai counterparts to confirm if any Australians have been affected by the attack.
One Australian tourist, John Murray, was across the road when the blast when off.
In an interview with Sky News, he said they were shocked by the power of the blast, but were unhurt.
"We were literally standing, looking into the shrine, and the explosion went off, it literally put a lot of pressure on our bodies, like our ears are ringing, we went to the floor, there was shrapnel everywhere, smoke, people screaming, we then ran down from the sky bridge where we saw it, and then there were people with parts of their legs coming off, blood all over the ground, and where the explosion went off there were just people everywhere, scattered."
Two suspicious packages were found by police ten metres from the initial blast site.
Both were remotely detonated.
Thai police chief, Somyot Poompummuang, says the blast was clearly intended to take as many casualities - locals and tourists - as possible.
"The blast radius of the bomb is about 100 metres. The bomb experts say that the bomb weighed about three kilograms. We haven't ruled out any motive. We are putting great importance on every motive."
Bangkok had been relatively peaceful since a military coup ousted a civilian government in May last year
Government spokesman, Major General Werachon Sukondhapatipak, has called for calm.
He says they are investigating who could be behind the blast.
"The public are advised to remain calm and monitor the situation regularly as the security agencies conduct their operations. As for possible motivations for those who may have caused this incident, it is too early to speculate which group may have been responsible for this crime, but authorities are following possible leads."
A spokesman for the military-backed ruling group, the National Council for Peace and Order, Sansern Kaewkamserd, has offered condolences to the families of the victims.
"The prime minister and the government would like to express our deepest condolences to the victims' families. The government insists that we will do everything in our power to investigate, with our utmost effort, to bring those involved and who are behind all of this to be sentenced."
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