Dozens of bodies wrapped in white cotton sheets lay on the pavement outside as fire crews moved in to douse the embers of the club in an area popular with foreigners and wealthy Thais.
Many of the bodies were said to have been burned beyond recognition.
"We were all dancing and suddenly there was a big flame that came out of the front of the stage and everybody was running away"
Several foreigners are thought to be among the dead, although their nationalities have not been released.
The arrival of rescue workers at the scene was reportedly delayed by Bangkok's heavy new year traffic.
The website of The Nation newspaper quoted one party-goer, Somchai Frendi, as saying the blaze was caused by "special effects'' fireworks to usher in the New Year.
The fireworks ignited the second floor ceiling, which was made largely of inflammable, sound-proofing material, he said.
|The fire quickly swept through the packed club, trapping revellers inside the burning building
The report also quoted a club worker as saying that explosions were heard shortly after the New Year countdown ended and someone shouted, "Fire!''
It is not clear whether the club's owners had adhered to city safety regulations, which are rarely monitored and loosely enforced.
Emergency workers said the deaths were caused by burns, smoke inhalation and injuries during a stampede to escape from the club, which had only one main door for entry and exit.
"We were all dancing and suddenly there was a big flame that came out of the front of the stage and everybody was running away," survivor Oh Benjamas told Reuters news agency.
"It was chaotic, too many people were stuck behind the door. Some of them tried to break through the glass window. I think a lot of those who died are people on the second floor who got stuck behind the crowd inside."
Watcharapong Sri-saard, a firefighter at the scene, told the Associated Press that another door at the rear of the building was known only to the staff.
He said a number of staircases inside the club as well as bars across the second-floor windows made escape difficult.