The CN Tower is now open following an early morning fire in the structure's antenna mast.

Staff at the tower were first alerted to the issue early this morning when an alarm was activated on the upper levels of the structure.

"Our security team responded to the alarm. It was shown that there was some sort of heat activation in the upper antenna," Neil Jones, Chief Operating Officer for the CN Tower, told CP24 outside the downtown landmark on Wednesday.

"We sent one of our electricians to have a look and there was a glowing on a large cable that is a cable for one of our broadcasters."

Staff notified Toronto Fire shortly after 4 a.m. and the fire department responded with seven fire trucks and nearly 30 firefighters. Members of the technical rescue team were also called in.

To get to the smoldering fire, crews had to climb up ladders to the antenna in an area located above the structure’s observation pod and below the tip of the tower, which stands 550 metres tall.

"The spot where the cable was is high up and it is also confined space so it’s very tight," Jones added.

Firefighters managed to extinguish the fire with CO2 at around 7 a.m.

Toronto Fire District Chief Stephan Powell said the call was "physically demanding" for the firefighters involved.

“It is something we do train for because it is a very unique building in the city of Toronto,” Powell noted.

Cleaning staff was evacuated from the building as crews worked to get the situation under control and no injuries were reported.

The cause of the fire is still undetermined but officials confirmed that it may be “of an electrical nature.”

"CN personnel, they are going to go in and see the extent of the damage," Toronto Fire Platoon Chief Kevin Shaw said Wednesday. "They have the people here already to repair it."

Jones said there was never any threat to public safety or the structure.

He said operations are running normally this morning at the downtown tourist attraction but crews will need to replace the cable that caught fire.

The incident created brief broadcasting transmission issues for some local media outlets on Wednesday morning.

"While we were accessing the space with Toronto Fire, we turned off some of the antennas to access the space safely… Once we were out of that area, then the antennas were turned back on," he said.

"Everything should be up and running."