"An electrical short circuit has caused the fire which has now been brought under control," police spokesman SSP Ajith Rohana said.
There were no deaths or injuries reported due to the fire.
All the equipment inside the structure were destroyed, Rohana said.
The temporary structure at the Bandaranaike Memorial International Conference Hall accommodated nearly 1,000 journalists both foreign and local during the three-day Commonwealth Heads of Government Meeting (CHOGM) here.
The police said the fire broke out this morning when an exhibition was being held in the temporary building.