British police arrested a group of suspected Islamic extremists, in a large anti-terror operation Monday.

Six men were apprehended during raids conducted in the central British city of Birmingham. Police say the men were were involved in the early stages of some kind of terror plot. They did not elaborate.

Authorities said it didn't appear that ongoing political conferences were the intended target.

West Midlands Assistant Police Chief Marcus Beale said the suspects were arrested in or near their homes by unarmed police officers as part of a "large, pre-planned, intelligence-led counter-terrorism operation."

A woman was also arrested for failing to disclose information. All seven suspects are between 22 and 32 years old.

The British national threat level remained at "substantial" Monday.

There are five levels of threat, from low, meaning an attack is unlikely, to critical, meaning an attack is imminent. "Substantial" is the third level.

"As more details emerge of this plot, the threat level may be reassessed," according to a British government official who spoke on condition of anonymity because of the sensitivity of his work and the ongoing investigation.