Seventy firefighters are tackling a "huge" blaze at the famous Camden Lock Market in north London.
Emergency services were called to the attraction shortly after midnight on Sunday after fire took hold in buildings near the Camden Stables.
The London Fire Brigade (LFB) said it had sent 10 of its fire engines to the area, while witnesses described seeing "lots of ambulances" racing to the scene.
The market is popular with tourists drawn to the area by the shopping and nightlife on offer.
Witness Joan Ribes, 24, said: "I was just passing by when I saw the fire and they started to get firefighters and police, it was all very fast.
"We called the police to close the street to the traffic because it was very dangerous, the fire was flying through the air to the surrounding areas.
"The fire was moving very fast. People were watching, but we were scared the building could explode at any time since there are restaurants with kitchens nearby."
A different part of the market complex was ravaged by a fire in 2008, leaving traders without a place to operate for months.
On Monday morning LFB tweeted that "the 1st, 2nd and 3rd floors, plus the roof, of a building within the market are alight.
"We now have ten fire engines and over 70 firefighters dealing with the #Camden Lock Market fire. Please avoid the area."
Footage posted on social media showed flames shooting into the sky while firefighters attempted to douse the flames.
One witness said: "Just drove past Camden market and there's a huge fire on top a building, close to the Lock Bridge! Crazy crazy scenes."
Another 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."
So far there have not been any reports of injuries from the blaze.
A London Ambulance Service (LAS) spokeswoman said: "We were called at 00.07 today (10th July) to reports of a fire at Camden Lock Market.
"We sent a clinical team leader and our Hazardous Area Response Team (HART) to the scene.
"We have not treated any patients, and we remain at the scene."
The Metropolitan Police said it had also been called to the scene.
The LFB said the cause of the fire was unknown at this stage.
The brigade added at around 3.20am that the fire "is now under control but crews will be damping down into the morning".