LONDON (Reuters) - British police said they had arrested four men on Wednesday on suspicion of preparing terrorism acts in the United Kingdom.

London police said officers from its Counter Terrorism Command had detained the four men, aged 18, 24, 25 and 27, at their homes in the east of the British capital.

“These arrests were pre-planned as part of an ongoing investigation by the Met’s Counter Terrorism Command and (the domestic spy agency) MI5,” police said in a statement. “The arrests are linked to activity in the UK.”

The men were being questioned at a south London police station “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism”, the statement added.

Britain has been on its second-highest alert level of “severe” since August 2014, meaning an attack by militants is considered highly likely.

In March, a man killed five people after driving into a crowd of pedestrians on Westminster Bridge in central London before stabbing a police officer in the grounds of parliament. He was shot dead at the scene.

Earlier this month police charged three women from London with preparation of a terrorist act and conspiracy to murder after a raid in which one of the women was shot by armed officers.

In another incident last month, a man was arrested on Parliament Street, near the parliament building and UK Prime Minister Theresa May’s Downing Street office, and later charged with terrorism and explosives offences.