A 30-metre cordon has been put in place around Castle Junction and members of the public have been advised to avoid the area.
A spokesman for NI Fire and Rescue said: “NIFRS are attending an incident in Belfast city centre. The call was received at 15:45hrs today and we have committed 24 fire firefighters on site to deal with the incident.
“This is a confirmed fire in underground electrical apparatus. NIFRS have established a 30 metre cordon for public safety which has closed Castle Junction. Two premises have also been evacuated due to smoke travel.”
The spokesman added: “NIFRS is working closely with Northern Ireland Electricity Service in order to bring the incident to a safe conclusion. I would encourage members of the public to avoid the area at this time to minimise traffic congestion.”