© PA
Seventy firefighters battled the blaze at at the Baitul Futuh mosque in Morden on Saturday afternoon, which London fire brigade said had hit admininstrative buildings and not the main prayer room.
One man was taken to hospital suffering from smoke inhalation.
Two boys aged 14 and 16 have been arrested on suspicion of arson, and both remain in custody at a south London police station, Scotland Yard said on Sunday.
Crews from fire stations across south London responded to the fire, which was reported by a member of the public just after midday.
Pictures on social media showed a column of black smoke billowing up from the mosque. Half of the building's ground floor was on fire, along with a quarter of its first floor and roof.
By mid-evening on Saturday, the fire had damaged 50% of the ground floor, according to LFB. Most of the damage occurred at the front of the building, while the prayer area of the mosque was unaffected.
Merton council leader Stephen Alambritis described the mosque as as "beautiful" and a "major landmark". He said damage would be a "big blow" to the community, adding that the mosque was a very secure building which is manned by security staff.
Siobhain McDonagh, the Labour MP for Mitcham and Morden, spoke of her shock at news of the fire. "The mosque does a huge amount for the local community," she said. "They are tireless raisers of funds for charity - their own charities' funds and charities in the community."
Police, ambulance services and 10 fire engines attended the scene and the A24 London Road was partially closed, causing traffic tailbacks. British Transport Police were at Morden South overground station, next to the mosque, where trains ran at reduced speed and passed through the station without stopping.
The vast Baitul Futuh mosque complex, which covers 21,000 sq m, was completed in 2003 at a cost of about £5.5m. It is said to be able to accommodate 10,000 worshippers.
Comment: The fire comes at an interesting time. If it was an arson attack, it just goes to show that some Londoners are taking the not-so-subtle propaganda hook, line and sinker. Despite pleas to the contrary, the narrative that the propaganda makers want accepted is that Muslims are evil, they are a threat, we are in danger, and something needs to be done about it. Of course, this will only lead to a disaster. See: Sadistic Saudi princes and Washington warmongers Vs Russia's civilizing force