"Vulnerable sites" in London will receive round-the-clock protection from uniformed police officers, after the latest attack on a building used by Muslims.

The Darul Uloom Islamic boarding school was "set alight by intruders" while 128 children and staff were inside, a spokesman for the school has said.

They were evacuated after the "suspected arson attack" at the school in Chislehurst, in the London borough of Bromley, at 11.50pm on Saturday.

Two boys were treated for smoke inhalation and 10% of the building was damaged during the blaze, which was brought under control within an hour.

It is the second suspected arson attack on a building used by the Islamic community since the murder of Lee Rigby in Woolwich on May 22.

A Somali cultural centre was burned to the ground in Muswell Hill, north London, in the early hours of Wednesday, and some Muslim groups have claimed police are not doing enough to protect people.

But Metropolitan Police Commissioner Bernard Hogan-Howe said extra resources would be deployed to sites deemed to be "at greatest risk".

"These are difficult times for London's communities," he said.

"The Met is now investigating suspicious fires at two locations within the Islamic community which have happened in the past few days. Fortunately no one has been hurt, but we know that fires can often prove fatal.

"So I want to reassure people that we are using our full range of policing tactics to protect sites that might be vulnerable. In all boroughs across London, there is an increased police presence around locations that might be at risk.

"We will maintain a 24/7 guard of uniformed officers at sites we consider to be at greatest risk.

"Detectives are working tirelessly to establish whether these fires were started deliberately, and if so, to catch those responsible.

"We should not allow the murder of Lee Rigby to come between Londoners. The unified response we have seen to his death across all communities will triumph over those who seek to divide us."

School adviser Saiyed Mahmood said: "The academic department of the school was set alight just before midnight by intruders. Initially the staff ... evacuated the students to safety.

"We are part of the British community and are deeply saddened by the events that have taken place last night and urge the community to stay firm and united in bringing the people responsible to justice.

"The community at large have to come together for a safe and peaceful life in Britain."

He added that the school has been vandalised a number of times in the past and police were aware of the incidents.

Detective Chief Superintendent Steph Roberts, Bromley borough commander, said no arrests had been made but appealed for calm and urged people not to speculate on the cause of the school fire.

"The fire is being treated as suspicious," she said.

The £3,000-a-year boarding school was established in 1988 with the purpose of producing "great scholars and Huffaz (people who have memorised the Koran) to preserve and transmit the eternal message of Allah".

The fires follow a rise in religious hatred crimes after the killing of Drummer Lee Rigby in Woolwich, the south east London, last month.

The letters "EDL" - the initials of the English Defence League - were found scrawled on the outside of the Somali centre attacked last week.