A hot yoga class in Melbourne got warmer than expected last night after a fire started in an underground storage area.

The fire broke out in a basement underneath a bikram yoga studio at Ascot Vale, in the city's north-west, just before 9:00pm.

A class was underway at the time and a number of people had to be evacuated.

"We were just going down into a backward bend and heard some noises," one woman who was evacuated said.

"The next minute there was a fire. That's hot yoga for you."

Bikram yoga is practiced in heated rooms. Classes often take place in rooms as hot as 40 degrees Celsius with 40 per cent humidity.

Hot Bikram Yoga Ascot Vale apologised to clients who were evacuated.

"Apologies to those evacuated this evening," a Facebook post read.

"Everyone is safe and no-one hurt."

In a separate incident, dozens of elderly residents had to be evacuated after a fire broke out in a unit block at Fitzroy North just after 9:00pm.

No-one was injured.