Brazilian Tourism Minister Celso Sabino told reporters at the venue that the fire was under control and no one was hurt but that he did not know whether delegates would be able to return on Thursday or Friday to the part of the venue where the summit negotiations were taking place.
Summit organisers also confirmed the fire was under control, adding that Brazilian fire officials had ordered the evacuation of the summit's entire premises.
The summit in the Amazon city was initially scheduled to wrap up on Friday but it missed a self-imposed Wednesday deadline to secure agreement among the nearly 200 countries present on issues including plans to phase out oil, gas and coal.
Developing countries are also pushing for a substantial increase in climate adaptation finance from industrialised countries, arguing that stronger support is essential as they face worsening storms, droughts, wildfires and floods.
The fire scare occurred in what is already a hive of activity during the summit's two-week run, interrupting ongoing negotiations inside the venue.
A siren sent delegates, observers and journalists running for the exits with their belongings as police lined up as a barrier to prevent anyone from nearing the area where the fire was reported.
TV footage showed flames and smoke inside the venue, a conference centre on the site of a former airport.
Sabino told journalists that the fire started near the China Pavilion, which was among several pavilions set up for events on the sidelines of the climate talks.
The fire quickly spread to neighbouring pavilions, said Samuel Rubin, one of the people in charge of an entertainment and culture pavilion.Â
He said nearby pavilions include many of the Africa pavilions and one aimed at youth.
Para state governor Helder Barbalho told local news outlet G1 that a generator failure or a short circuit in a booth may have started the fire.
Since kicking off earlier this month, the summit has drawn multiple protests demanding action on climate change and forest protection, at times interrupting talks.
with AP and DPA
