British police on Monday were questioning four men in their 20s arrested in London on suspicion of terrorism.

The London police firearms unit took part in the arrests. British police rarely carry weapons, and their involvement suggests concern the men might have been armed.

The Metropolitan Police force said that the four were arrested during a pre-planned, intelligence-led operation on Sunday, on suspicion of the “commission, preparation or instigation of acts of terrorism.”

They did not identify the suspects, or give details of the charges.

Police said officers were searching six locations in London following the arrests.

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