Staff at Carlton's iconic La Mama Theatre have vowed to rebuild after a fire which destroyed its Faraday Street base on Saturday morning.

Firefighters were called to the building at about 5:00am to find fire had already burned through the roof.

Assistant chief fire officer Mark Carter of the Metropolitan Fire Brigade (MFB) said it took 50 firefighters about 45 minutes to bring the "very intense" blaze under control.

Commander Carter said while firefighters had been able to prevent the fire spreading to neighbouring properties, some shops on Lygon Street needed to be closed for the day because of fears the theatre building could collapse.

The MFB are using a drone to assess the structural integrity of the building.

"What's really sad about this, it's a heritage listed building, it's well-loved, it's been a business that's operated in the community for a really long time and that's evident by the scenes out in the street with a lot of people crying, a lot of people very emotional about the loss of this icon," Commander Carter said.

The Metropolitan Fire Brigade's Mark Carter said the blaze was "very intense". ( ABC News: Danielle Bonica )

'I'm completely gutted'

One of those surveying the damage on Saturday morning was La Mama chief executive Liz Jones, who has been involved with the theatre company for 40 years.

"The building is completely gutted, and I'm completely gutted," she said.

For more than 50 years La Mama has produced cutting edge contemporary theatre, with alumni including Cate Blanchett, Julia Zemiro and David Williamson.

"It's where so many people who are now household names cut their teeth, but more importantly than that again, it's like where most people in theatre got their start, or passed through it in some way," Ms Jones said.

"It's a very small space but it had a very big history and a very, very important place in the heart of the creative community of Australia really, not just Victoria."

She said the company would rebuild.

"Support is just pouring in from people — people can't believe it."

Two people embrace in front of La Mama Theatre in Faraday Street Carlton, which was destroyed by fire on May 19, 2018. ( ABC News: Danielle Bonica )

Hayley Fox, La Mama's venue technical manager, said the destruction of the theatre was "absolutely heartbreaking".

"We had our 50th celebration just last August and the community just came together like it was someone's grandma celebrating a big milestone.

"It's where so many people have gotten their start, whether you're an actor or director or writer, La Mama gives everyone the chance to just start.

"We'll come back, we'll rebuild, and we'll give thousands more people a chance to start, but right now it feels a little bit difficult."

La Mama Theatre staff Hayley Fox (L) and Amber Hart (right) are devastated by the loss of the building. ( ABC News: Yvette Gray )

'We need to make a plan'

Front of house manager Amber Hart said she was "absolutely devastated" by the loss but needed to be practical.

"Obviously there's a show that's supposed to go on tonight and so we're just going to have to figure out a plan for the next month or whatever," she said.

"There's a lot of people booked in to see these shows, and a lot of work's gone into it, so now we need to make a plan to see what we can do to maybe salvage something or even get it up running somewhere else for the time being so we don't lose all that work."

Ms Hart said she had worked at La Mama for 10 years and the theatre was "pretty much my life".

"I see so many amazing things here and meet so many great people and see how much La Mama transforms them. It's like a big warm hug, so I look forward to getting back to that.

No one was injured in the blaze.

The cause of the fire has yet to be determined.