Construction workers fled a building which was undergoing renovations in Melbourne's CBD after it caught fire Monday morning, putting in grave danger the lives of those on-site.

Metropolitan Fire Brigade Commander Lou Mele says the workers made such a "lucky escape" considering the fire could easily get worse in a construction site.

"It's a very lucky escape considering it's under construction and it's a good 70 feet above ground," he said.

The fire reportedly started when sparks from a welder showered electrical wiring on the seventh floor of the building.

The blaze was put under control within half an hour.

Authorities told the media no serious injuries were reported, except a man was receiving treatment for smoke inhalation.