The fire started at the Centara Grand hotel and at the Central World plaza, where people are said to have jumped from buildings. Dramatic images and videos show thick black fumes floating from the roof of the convention center. The 57-storey Centara Grand is the flagship flower of the central group and one of the largest hotels in the capital of Thailand.
The fire broke out between 17:00 and 18:00 local time (11:00 and 12:00 BST).
Traffic is blocked around the center while the emergency services try to reach the area during rush hour.
Rajdamri Road had stopped when hundreds of people had gathered outside after being evacuated from Central World.
A witness said on Twitter: "Run away from the dreaded Central World, the seventh floor of the fire is very dense smoke, but there are no alarms.
"Now I'm worried about people in the cinema because there are no announcements yet."
The Pathum Wan district office said the fire started on the fourth floor of the Centara Grand Hotel and the Bangkok Convention Center.
They are studying the cause of hell.
CentralWorld and the hotel are located in central Bangkok in the Pathum Wan district, one of the most popular tourist areas, known for its shopping malls frequented by tourists and locals.
CentralWorld was burned down in 2010 when anti-government protesters known as the Red Shirts were released from the center after months of demonstrations.
Bangkok Sky Train operators have stated that the nearby Sian station and Chidlom station are still in operation.