"We reaffirm our position that we clearly separate politics and sport. Thailand is ready to organise the torch relay peacefully and orderly," Sanan Kachornprasart, Thailand's deputy prime minister, said at the ceremony.
A Western woman carrying a picture of the Dalai Lama, the Tibetan spiritual leader, was at the scene and reported as the sole dissenter.
At the UN headquarters, 3km from the torch's starting point, about 60 pro-Tibet protesters gathered waving banners reading: "Free Tibet," "Stop Killing in Tibet" and "No Torch in Tibet."
Modern tradition of torch relay began in 1936 at Berlin Olympics
This year's is the longest ever, travelling 137,000km
The torch will visit 19 nations over 130 days
First destination is Kazakhstan
On the final leg, it will travel throughout China, including Tibet and the summit of Mount Everest
Click here to go to the official torch relay site
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They were matched by about 60 pro-Chinese protesters in red T-shirts, waving the Olympic flag and shouting "China, come on!" and "Olympics, come on!"
Police separated the two groups.
Authorities are trying to keep the relay free of the protests that hit the London and Paris legs earlier this month.
Thai police have warned they will arrest, prosecute and deport any foreign nationals breaking the law to protest Beijing's human rights record.
The torch arrived in Thailand on Friday, escorted by Chinese security officials, on a charter flight that landed at a military airport outside Bangkok.
The next stop on the torch's journey is Kuala Lumpur, the Malaysian capital.
There authorities will deploy an elite police squad, alongside 1,000 police officers, to protect the torch, Malaysia's The Star newspaper reported on Saturday.