A huge blaze broke out overnight at Camden Lock Market in north London.
Seventy firefighters and 10 fire engines were sent to the site, which is a popular tourist attraction, London Fire Brigade (LFB) said.
A fire officer at the scene said the fire began in a building containing a number of businesses.
It affected a small section of the area and many stalls and shops were "operating and welcoming visitors and customers as usual", the market said.
London Ambulance Service was called in, but confirmed it had not treated any patients.
Major fires have hit the sprawling market area twice before in recent years, in 2008 and 2014.
LFB said the first, second and third floors and the roof of the building had been damaged.
"Crews worked hard to get the fire under control and to stop it from spreading to neighbouring buildings," station manager David Reid said.
BBC Radio London reporter Barry Caffrey said he had been told by a fire officer at the scene the blaze had begun in a building with an Honest Burger restaurant on the ground floor.
Deana Irwin, who lives next to the market, saw flames blazing "about 5m high from the top of the building".
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 market, which has been based in the area since 1974, has more than 1,000 stalls and shops.
Deicola and Leora Neves, who own Camden Guitars which is based in the burnt building, said their shop had been destroyed and "everything has gone".
"This is where we started and we're really feeling the loss of that," Ms Neves said.
Market worker Kareem Khodeir said he believed about 100 traders would have been affected by the blaze and some would be "finished" as a result.
"There were 30-35 permanent stalls in the building who have completely lost everything while those who trade outside also store their stock in there.
"It most likely will destroy a few businesses completely," he said.
Alex Proud, founder of the Proud Galleries in Camden, said the fire brigade had "turned up incredibly quickly and stopped what could have been a really substantial fire which could have wiped out the market".
"These are old buildings, they go back to the 1840s," he added.
He said only about 5-10% of the market had been damaged and "75% of the market is now open".
At the scene: Jennifer Scott, BBC News
It's a tourist hotspot that attracts millions and an area of the capital with rich rock 'n' roll history.
But Camden Lock Market is also the livelihood of many small businesses and a stone's throw from local residents who had to watch as the fire took hold.
Nestled next to Regent's Canal and beside Camden's railway bridge, visitors have vast quantities of shops to explore and cuisine to sample.
Now the Market Hall - a four storey building filled with independent traders who often make their own products - looks badly burnt, with broken windows and a strong smell of smoke surrounding it.
"The hard work and aggressive action of the fire crews ensured it didn't spread to the nearby buildings," said London Fire Brigade station manager Andrew Walton.
Ozgur Kaya works on a jewellery stall in the building.
"Some of the traders have lost everything," he said. "They are so upset. We are all here to be there for them."
Sam Row, who runs a vintage camera stall in the shadow of the building, only discovered the news when he came to work first thing.
"All my kit is in there," he said. "I don't know if it is safe, if it has been damaged by fire or water. It's very worrying for us."
The ambulance service said it sent a clinical team leader and a Hazardous Area Response Team.
At about 03:00 the LFB said the blaze was "now under control but crews will be damping down into the morning".
In a statement, the brigade said: "Four fire engines and around 20 firefighters will remain at the scene on Monday."
The cause of the blaze is unknown.
On 8 February 2008, the famous celebrity haunt The Hawley Arms was severely damaged in a blaze, along with six shops and 90 market stalls.
In 2014, some 600 people fled a blaze in the Stables Market.
The market, located in the former Pickfords stables and Grade II-listed horse hospital, burned for two hours on the evening of 20 May.