London (CNN) -- Police arrested six men in Birmingham, England overnight in what they called a pre-planned, intelligence-led counter-terror operation, they said Monday.

West Midlands Police then arrested a woman Monday morning on suspicion of failing to disclose information which might prevent an act of terror, they said.

Assistant Chief Constable Marcus Beale said the arrests were necessary "in order to ensure public safety."

He declined to go into details about what the men were suspected of, saying "the operation is in its early stages."

Britain did not change its terror threat level in light of the arrests, leaving it at "substantial."

The six men, aged from 25 to 32, were detained by unarmed officers, the police statement said.

Deputy Prime Minister Nick Clegg's Liberal Democrat party is holding its annual conference in Birmingham, but police said the arrests were not linked to the political event.

Police are searching the homes of the suspects and other locations in Birmingham, Britain's second-largest city, they said.

CNN's Andrew Carey contributed to this report.