LONDON (Reuters) - British police have arrested four teenagers in connection with a fire at an Islamic school that has heightened fears of anti-Muslim reprisals after a soldier was murdered on a London street last month.

London's Metropolitan Police said they would increase their presence at places considered at risk and would maintain an around-the-clock guard at some locations.

"We should not allow the murder of (soldier) 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," Police Commissioner Sir Bernard Hogan-Howe said on Sunday.

The fire on Saturday followed a suspected arson attack on an Islamic center in north London on Wednesday. "EDL" was found scrawled on the side of that building.

EDL is the acronym for the English Defence League, a far-right group that has held several protests since Rigby's murder. It denied involvement in Wednesday's fire.

(Reporting by Sarah Young; Editing by Robin Pomeroy)