More than 120 pupils and staff were evacuated from Darul Uloom School, in Foxbury Avenue, Chislehurst, yesterday. Two men were treated for the effects of breathing in smoke.
"The fire was extinguished shortly after the arrival of London Fire Brigade. Enquiries continue into the circumstances of the fire. At this early stage it is being treated as suspicious," a Metropolitan police spokesperson said, adding that no arrests have been made so far.
London Fire Brigade said there was minor damage to the building and 21 firefighters tackled the blaze.
Last week, a Somali cultural centre and mosque was burned to the ground in Muswell Hill, north London, and the letters "EDL" - the initials of the right-wing English Defence League - were found scrawled on the outside.
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The 3,000 pounds-a-year boarding school was established in 1988 with the purpose of producing "great scholars and Huffaz (people who have memorised the Quran) to preserve and transmit the eternal message of Allah", according to the school's website.
The Council on American-Islamic Relations (CAIR) has discussed its revamped security regulations with the UK's Muslim Council of Britain (MCB) in light of the murder of Rigby.
Fiyaz Mughal, director of the conflict resolution charity Faith Matters, said too many mosques remained vulnerable to attack in the aftermath of Woolwich.
He warned that, of the UK's 1,500 mosques, 1,300 urgently needed to improve security.