West Midlands Police said the flames seen on New Street shortly before 17:00 GMT were caused by an electrical fault below.
West Midlands Fire Service said it had been liaising with power suppliers to deal with the problem.
There were no reports of injuries, police said.
Shoppers and workers who were about to make their way home shared images of the scene, where tram travel was disrupted.
The fire service said it would tackle the blaze from within a service hatch, once the electrics were isolated by engineers.
A Western Power Distribution spokesman said 103 properties had been affected by a power outage as a result of a fault with a junction box.
He added that power had been restored to 60 properties by 19:00 GMT, and the remaining properties should have their power restored by 01:30 on Saturday.
Comment: There has been a huge surge in underground urban fires in the last decade or so.
This, for example, happened in England in 2015: Film footage shows fireball exploding from underneath street in Shirley, UK
In 2014, the British govt suggested that the increasingly extreme levels of rainfall was to blame, with flooding causing short-circuiting of electrical cables.