Toronto crews have extinguished a fire that started inside the broadcasting antenna atop the CN Tower early today.

Anyone with over-the-air television feeds and radio channels may be affected by the antenna outage, according to the Toronto fire department. The antenna is responsible for broadcasting more than 30 television and FM radio signals across southern Ontario, the tower website says.

Some 30 firefighters were on the scene after the fire started around 4:15 a.m. ET. An electrician noticed a signal on the alarm panel that monitors the antenna, Neil Jones, CN Tower's chief operating officer, told CBC Toronto.

A fire in the broadcasting antenna of Toronto's CN Tower started around 4:15 a.m. Wednesday. (CBC)

"It looks like a tar substance that's burning," Toronto Fire Platoon Chief Kevin Shaw said before the fire was put out hours later.

Toronto Fire District Chief Stephan Powell said: "The mast itself is steel. It's above the SkyPod. There's numerous pieces of conduit that feed the various antenna feeds that are within there and one of these have started to maybe smoulder or burn."

When I heard it at first, I just continued to do my job because I didn't take it seriously. - Jenive Grant, owner of cleaning company

No injuries were reported and cause of the fire isn't clear.

The overnight cleaning staff were evacuated from the tower when the fire was discovered, Jones said.

Jenive Grant, owner of a cleaning company contracted by the CN Tower, says she was shocked when she heard the fire alarm go off. (CBC)

Jenive Grant was cleaning the basement kitchen when she heard the fire alarm go off.

"When I heard it at first, I just continued to do my job because I didn't take it seriously, but then afterwards when they said exit, and I'm like 'woo,' so I just grabbed my stuff and left," said Grant, owner of a cleaning company.

'Like being inside a silo'

While crews train for situations like this, Powell said there are challenges while trying to extinguish a fire within a confined highrise space.

"It's like being inside a silo. There's a ladder there that runs up the interior of the transmission tower, so there's not a lot of room to work up there," Shaw said.

The antenna is above the lookout and restaurant levels of the tower. According to the tower's website, the antenna consists of 44 pieces and is 102 metres high.

Jones said one cable within the antenna was down. It's not clear which broadcaster is affected by the outage at this time, he added.

Crews de-energized the mast and shut off power to the antenna, Powell said, as other wiring within the mast were at risk of catching fire.

Jones said the tower's observation decks and 360 restaurant will be open this morning.