The live blog has closed off. See our main article for all information on the situation in Bangkok.
8.49am: The British Foreign and Commonwealth Office said: "We are in contact with local authorities following an explosion in Bangkok. At this time we have no reports of British nationals killed or injured but we are urgently seeking further information."
8.45am: Australian Department of Foreign Affairs and Trade says there's no information that Australians are among the severely injured or killed.
8.38am: A British tourist who was in a hotel directly above the bomb blast has told Sky News about the moment the bomb went off.
Lucinda-Jane Chastain said, "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."
8.33am: The Erawan shrine, where the bomb was laid, is on a busy corner near five-star hotels, busy shopping centres, offices and a hospital. It is a major tourist attraction, and a place where Thai people go for worship.
8.30am: In Auckland, Thai Society president Songvut Manoonpong said parties opposing the Thai military government were likely to have been responsible for the blast.
"They try to discredit the military government," he said. "This area is a very popular place and if it happens the world will know."
He was shocked when he saw the news in an email.
"It's a big surprise. It's very bad," he said.
"I think the government can do nothing at the moment because they don't know who has caused this bomb blast. When they know, they can do something."
Warning: Graphic content:
8.28am: Jonathan Head of BBC tweeted: "The sight of those terrible injuries after the Bangkok bomb will be with me for a long time. A cruel, calculated attack - but who? and why?"
8.26am: It is now being reported that among the dead are one Filipino and two Chinese nationals
8.24am: Canadian Derek Van Pelt has lived in Bangkok for 17 years.
"It was absolute craziness. There was broken glass everywhere, there were body parts everywhere," Van Pelt told Toronto's the Star newspaper in a phone interview from the Thai capital.
He said he saw three motorcycles in the intersection when he arrived and "one of them appeared to still be smoldering."
8.19am: Dashcam footage has emerged on Facebook of the moment the bomb exploded. Fiery debris can be seen being flung into the air above the shrine.
8.10am: International media have been tweeting live from the scene.
8.05am: India's Prime Minister Narendra Modi issued a statement after the explosion. "I strongly condemn the blast in Bangkok. My thoughts are with the families of the deceased. I pray for a speedy recovery of the injured," he said via Twitter.
8.00am: The bomb explosion targeting tourists has been called "deja vu" for Bangkok's tourism industry.
7.56am: The nationalities of the people killed in the blast are still not confirmed.
7.53am: Kiwi Gulpreet Fletcher is in Bangkok was heading to the shrine when the bomb went off. "Thank God we were running late... we could have died.
"My whole family came here for a visit. We got there a few minutes after...It was scary."
7.49am: Tamar Johnson, 20, a British tourist told Cater News Agency she and her sister were at the rooftop bar near the bomb site when they heard a huge explosion. "At first we thought it was just thunder because there's been electrical storms all day so we didn't think much of it.
"Then my mum phoned me from the hotel. She was shouting about a bomb so we sprinted back to our hotel. It was chaos - people were running around and there were police and ambulances everywhere."
7.45am: Picture from the scene:
7.40am: Defence spokesman Kongcheep Tantrawanich described the attack as "the work of those who have lost political interests and want to destroy the 'happy time' of Thai people". "It's an attempt to ruin Thailand's tourism image and cause damage to the country's business sector,"he said. "The group behind the bomb must stop the savage act done to their fellow Thais and stop hurting the nation."
7.34am: All 438 city schools have been closed for safety reasons, the Bangkok Post reported.
Here's some raw footage from the scene of the blast:
7.24am: The Bangkok Post said most of those injured in the explosion were said to be predominantly Asian tourists.
7.20am: In Bangkok, police said the IED that exploded inside the shrine area was composed of three kilogrammes of TNT stuffed in a pipe and wrapped with white cloth. Its destructive radius was estimated at 100 metres.
7.14am: We are five hours ahead of Thailand, so more information will be coming as daybreak arrives in Bangkok.
6.57am: Mfat warned there were unconfirmed reports of other bombs found nearby.
Here's a still from security footage of the bomb explosion.
6.45am: Here's what Mfat sent to Gaffaney and other Kiwis in Bangkok:
"New Zealanders in Bangkok are advised to avoid travel to downtown Bangkok for the time being.
"We recommend exercising heightened vigilance at this time, monitoring the media for updates and adhering to the instructions of local authorities at all times, including any restrictions on movement."
NZME News Service reporter Catherine Gaffaney went to the site of the explosion soon after the bomb went off. She said the area was closed off with military personnel. She said the tourists in her hostel were feeling quite nervous.
6.22am: NZME News Service reporter Catherine Gaffaney is in Bangkok. She says the streets are empty and the mood very sombre. We are talking to her this morning to get further information from the scene.
6.14am: Bangkok has never been hit by such a deadly attack.
6.11am: We are trying to reach Ministry of Foreign Affairs and Trade right away to confirm whether any New Zealanders were hurt or killed in the blast.
6.09am: Latest update from Thai police is that there are 123 injured in the blast.
6.07am: "We still don't know for sure who did this and why," Deputy Prime Minister Prawit Wongsuwon told reporters. "We are not sure if it is politically motivated, but they aim to harm our economy and we will hunt them down." Read the full story here.
6.05am: The bomb exploded in Bangkok's Erawan Shrine during rush hour, an area that is popular with tourists. No one has immediately claimed responsibility for the bomb.
6.03am: A New Zealand paramedic working with the Bangkok ambulance service, Marko Cunningham, said the blast had left a 6ft crater.
"There were bodies everywhere. Some were shredded. There were legs where heads were supposed to be. It was horrific."
6.01am: Good morning everyone. Terrible news overnight from Bangkok, a large bomb has exploded in Bangkok, killing 18 people. Here's the full story.