The British police said Monday that officers arrested six men and a woman overnight on terrorism-related charges in the central city of Birmingham. The arrests followed a “large, intelligence-led counter-terrorism operation,” the police said.

The men were aged between 25 and 32 and the woman, accused of failing to disclose information, was 22, the West Midlands Police said in a statement.

The men were arrested by unarmed officers in raids in five neighborhoods between 11:00 p.m. Sunday and 1 a.m. Monday, while the woman was detained at 6:30 a.m. Monday. The BBC said the investigation, believed to have drawn in Britain’s MI5 domestic security service, had uncovered links to Islamic militancy and was part of the most significant counterterrorism operation so far this year.

A West Midlands police official, who requested anonymity because he was discussing a still unfolding operation , said that initial speculation the arrests were linked to Irish republicanism was unfounded. The official declined to confirm the BBC report that those arrested were Islamic extremists, but said the operation was “linked to international cases as much as local.”

The police statement said the men were arrested “on suspicion of preparing or instigating an act of terror,” and police were searching their homes and seven other locations in Birmingham, Britain’s second-largest city which has an ethnically mixed population. The arrests took place in a range of areas from impoverished inner city neighborhoods to comfortable suburbs. 

Marcus Beale, the West Midlands assistant chief constable, declined to elaborate on the nature of the suspected attack. “The operation is in its early stages so we are unable to go into detail at this time about the nature of the suspected offenses,” he said in a statement. “However, I believe it was necessary to take action at this time in order to ensure public safety.”

Britain’s Press Association news agency said the arrests were apparently unrelated to the annual party conference of the Liberal Democrats, the junior partner in Prime Minister David Cameron’s coalition, which is under way in Birmingham.

The arrests were the first of their kind reported since April when five men were arrested near the Sellafield nuclear fuel reprocessing plant in the north of England under antiterrorism laws. They were released later.

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