A large fire has engulfed the ground and first floors of a south London mosque which is said to be the largest in western Europe.
Seventy firefighters are tackling the blaze at the Baitul Futuh mosque in Morden and a man has been taken to hospital suffering from smoke inhalation.
Shocked onlookers gathered opposite the mosque, watching a dark plume of smoke billowing out of the building's roof, before they were ushered behind a police cordon.
Police, London Ambulance and 10 fire engines attended the scene and the A24 London Road was partially closed, causing traffic tailbacks.
Phil Morton, station commander for Croydon fire station, said the cause of the blaze is still unknown.
"We are working very hard to control the fire but, as it's such a big building with such high ceilings, I imagine it will take a while yet," he said.
"We are using an aerial appliance to apply water to the roof - a fire break to stop the fire spreading.
"We expect a large amount of the building to be damaged."
He added that a large proportion of the front of the building was initially ablaze and that crews had worked hard to put it out quickly.
A London Ambulance Service (LAS) spokesman said: "We treated a male, reported in be in his 40s, with smoke inhalation and took him to St George's Hospital as a priority."
British Transport Police were at Morden South overground station, adjacent to the mosque, where trains were running at reduced speed and passing through the station without stopping.
A member of the public reported the blaze at 12.06pm.
The mosque was built for the Ahmadiyya Muslim community to provide people with a meeting place and somewhere to hold social religious events. It was built on the site of the old Express Dairies in 1999. The building bears the Ahmadiyya Muslim motto "Love for all, hatred for none".
Merton Council leader Stephen Alambritis described the mosque as a "major landmark" in the area.
Damage to the "beautiful building" will be a "big blow" to the community, he said.
Mr Alambritis said the mosque is a very secure building, which is manned by security staff, and suggested the fire could have been caused by cooking preparations rather than an arson attack.
He said: "I know from the many times I have been there how well used and professionally it is run.
"Every time I have been there, I have been met at the doors and the people are very visible and friendly.
"I could not see how any kind of arson could take place but that is for the police and the fire brigade to investigate. I am sure that this community will want to know themselves whether there is any possible arson.
"It is a very beautiful building and what has happened is very sad. There were no people at prayer at the time (the fire broke out). It does seem that evacuation procedures worked.
"The congregation there, especially on a Friday, are very large and peaceful and they add very much to the community. This will be a blow to their operation but they will be back."
The local Labour MP for Mitcham and Morden, Siobhain McDonagh, spoke of her shock at the news of the fire.
"The mosque does a huge amount for the local community. They are tireless raisers of funds for charity - their own charities' funds and charities in the community," she said.
"Their motto is 'Love for all, hatred for none' and they really live that. I have been to the mosque many times - it is a beautiful building and my heart goes out to them."
Lord Ahmad, a high-profile member of Ahmadiyya Muslim community and junior government minister, tweeted that "this is a beautiful mosque complex which has transcended barriers and been a beacon of peace - thoughts & prayers".
Ten fire crews and one specialist fire unit remain on the scene to try to calm the fire which has damaged 50% of the ground floor, London Fire Brigade said.
Most of the damage is thought to have occurred at the front of the building, while the prayer area of the mosque is thought to be unaffected.
Only a handful of worshippers were inside the mosque when the blaze started and were evacuated from the site.
Mr Morton said: "We are working really closely with the local community and members of the mosque and we are trying to identify key areas of the building to try to protect.
"I imagine for the local community this is quite significant."
Fareed Ahmad, national secretary for external affairs for the Ahmadiyya Muslim Community UK, said: "It's a very difficult time and obviously we're praying that it comes to a safe conclusion.
"This is a devastating accident to happen to any mosque.
"What we understand at the moment is that the mosque itself where the prayers take place at the back of the site is safe and untouched by fire."
The mosque is situated between the Northern line Tube depot and Morden South overground station, which could lead to travel disruption later.
The Ahmadiyya community launched a multimillion-pound campaign in February 1995 which funded the building of the mosque. The dome stands 75ft (23m) above ground level.
The 5.2 acre (2.1 hectare) complex was completed in 2003. There is space for at least 10,500 people to pray, according to the mosque's website.
The fire is affecting around half of the ground floor, part of the first floor and a section of the building's roof.
London Fire Brigade later said the blaze had hit admin buildings and that the "mosque itself is thankfully unaffected".