The fire service said it is too early to say where the blaze started or whether anyone has been injured.
A Metropolitan Police spokeswoman said: "Police were called on Monday at 12.10am to Camden Lock Market to reports of a fire. London Fire Brigade were already at the scene when officers arrived. It is unknown at this stage if any persons are injured, we await update."
However witnesses say they believe the fire started at Gilgamesh - an upmarket Indian restaurant based inside the huge market hall, home to 1000 businesses.
Part of Camden High Street has been blocked off while crews battled to get the fire under control with members of the public are urged to stay clear of the area.
Speaking to MailOnline a spokesman for London Fire Brigade said they had not increased the numbers of firefighters on the ground.
He said: "We now have 10 fire engines there and 70 firefighters. We were called just before midnight.
"There are no reports yet of injuries but we were only called half an hour ago.
"It is an ongoing incident and there will be updates through the morning."
People in the area tweeted images as the blaze erupted.
Footage posted on social media showed firefighters dousing a significant fire.
One witness tweeted: "On a night bus I just passed the start of a big fire in Camden, now at Euston I've seen lots of ambulances head north."
Another said: "Just drove past Camden market and there's a huge fire on top a building, close to the Loch Bridge! Crazy crazy scenes."
A major blaze ravaged the area in 2008, shutting the attraction for several months.
The fire in Camden comes almost a month after a ripped through Grenfell Tower in North Kensington, leaving at least 80 dead.
This is the second time in a decade the iconic market, to the east of Chalk Farm Road has caught alight.
The area was hit by a major fire in 2008, shutting the attraction for several months.
And just days ago Camden councillor Georgina Gould tweeted that enhanced fire checks were being carried out in tower blocks across the area following the Grenfell Tower tragedy.
- Telegraph UK