Four suspects – two aged 17 and two aged 18 – arrested on suspicion of arson at Darul Uloom school in Chislehurst

Four teenagers have been arrested in connection with a fire at an Islamic boarding school on the outskirts of London. The four suspects – two aged 17 and two aged 18 – were detained on Sunday night on suspicion of arson at the Darul Uloom school in Chislehurst. The blaze is being treated as suspicious and the four remain in custody at a south London police station.

The arrests came as the Metropolitan police commissioner, Bernard Hogan-Howe, revealed that police had stepped up security at other potential targets around the capital, with uniformed officers on guard at key sites amid fears of reprisals after the murder of Drummer Lee Rigby. There have been a string of attacks since the soldier was hacked to death in south-east London on 22 May in a suspected Islamist terrorist attack.

The emergency services were called to the school as pupils and teachers slept on Saturday night, and two boys were treated for smoke inhalation after the fire broke out but did not need hospital treatment. The emergency services were called to the school at about 23.50pm and 128 pupils and staff were evacuated.

Firefighters extinguished the blaze soon after and pupils were able to go back into the school the following morning. There was minor damage to the £3,000-a-year school.

Hogan-Howe said security around the capital had been stepped up in the wake of several attacks but urged all communities to shun violence.

"We should not allow the murder of Lee Rigby to come between Londoners," he said. "These are difficult times for London's communities. The unified response we have seen to his death across all communities will triumph over those who seek to divide us."

The police chief said the force was using its full range of policing tactics to protect potential targets. "In all boroughs across London, there is an increased police presence around locations that might be at risk," he said. "We will maintain a 24/7 guard of uniformed officers at sites we consider to be at greatest risk."

Saiyed Mahmood, an adviser to the school, said: "The academic department of the school was set alight just before midnight by intruders."

He urged the community to come together to help police trace the culprits. "The parents should rest assured that the students are safe and have been well looked after. Initially the staff extinguished the fire and evacuated the students to safety," he added. "Two boys suffered smoke inhalation injuries, but they are recovering."

Darul Uloom is about six miles (10km) from Woolwich, where Rigby was killed last month. An unnamed uncle of a 13-year-old pupil at the school said: "I'm really worried about it. I've been living here for the past 40 years and we've never had anything like this." Speaking to Sky News, he added: "We are not free at the moment."

The school was established in 1988. Its website says it aims to prepare Muslim students to be good Muslims and responsible citizens; to embed in the student a sense of discipline; to enable them to grow up to become upright, respectable and worthy citizens of their respective countries.

Mahmood added: "The school is open to anyone who wishes to have a better understanding of what we teach. We are part of the British community and are deeply saddened by the events that have taken place 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."

The fire at the school comes as police continue to investigate a fire that destroyed an Islamic centre in Muswell Hill, north London. Graffiti linked to the English Defence League was found on the outside of the building. The far-right group denies involvement.

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Faith Matters, a group that monitors anti-Muslim hatred, has reported a sharp increase in incidents, including attacks on 11 mosques, since Rigby was killed.