At least 18 people have been killed after a bomb exploded in central Bangkok, emergency services have said.
Media in the Thai capital suggest the death toll could be as high as 27 and that four foreigners are among the dead.
The blast happened near the Erawan Hindu shrine in the downtown Chidlom district - a shopping hub popular among tourists.
At least 117 people have been wounded, some of whom have lost limbs.
A second bomb inside the shrine was reportedly discovered and defused, while there are reports of a third device on a train that teams have now neutralised.
CCTV footage showed a huge orange fireball as the bomb exploded and people fleeing down the street.
Witnesses described seeing bodies "shredded" in the street, while locals said the explosion sounded similar to a loud thunderclap.
Some reports suggest the bomb was inside the shrine, while others say it was on one of six motorcycles seen strewn across the street in the aftermath.
Many of those wounded are from China and Taiwan, local media say.
No-one has claimed responsibility for the attack.
One government minister said he believes the attackers chose the shrine to damage the tourism industry and the economy.
Deputy national police chief Aek Angsananond said: "All I can say now is there has been an explosion in central Bangkok involving a motorcycle bomb."
Former Bangkok resident Lucinda-Jane Chastain, who was having cocktails in a hotel above the blast site, told Sky News: "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."
Details of the bomb are now starting to emerge.
Terrorism expert Joe Gyte told Sky News: "The bomb has been confirmed to be a TNT-based improvised explosive devise.
"The streets are chaotic so it is quite difficult to get more information from the scene."
The Department of Foreign Affairs say they are aware of the situation, but have "no confirmation of Irish casualties".
People concerned can call the department on 01-408-2000.
The embassy in Bangkok is also asking those who are in the city to contact people to let them know they are safe:
The Embassy would urge Irish citizens to contact their family at home to let them know they are safe. Our contact number is 02-126-8092-7.
— Irish Emb Bangkok (@IrlEmbBangkok) August 17, 2015
One government minister said he believes the attackers chose the shrine to damage the tourism industry and the economy.
Deputy national police chief Aek Angsananond said: "All I can say now is there has been an explosion in central Bangkok involving a motorcycle bomb."
Bangkok resident Thomas O'Malley said: "I was jogging and I felt a really large explosion. I rounded the corner where the busy shopping malls are and saw the smoke, fire, absolute chaos, and a fireball".
"There were body parts in the street".
The US State Department said it was too soon to tell if the blast was a terrorist attack.
State spokesman John Kirby said authorities in Thailand were investigating and had not yet requested assistance from the US.
Deputy Prime Minister Prawit Wongsuwon told reporters: "We still don't know for sure who did this and why.
"We are not sure if it is politically motivated, but they aim to harm our economy and we will hunt them down."
Former Bangkok resident Lucinda-Jane Chastain, who was having cocktails in a hotel above the blast site, said: "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".
The shrine - which is popular with tourists - sits at the Rajprasong intersection which has been the centre of many political demonstrations in recent years.
The army has ruled Thailand since May 2014, when it ousted an elected government after months of violent anti-government protests.
