LONDON - British police arrested a group of suspected Islamic extremists on Monday in one of the most significant counter-terror operations of the year.
Officers were also carrying out raids on several homes and a commercial property in the central city of Birmingham as part of the intelligence-led operation.
Although the plot was in its early stages and targets weren't immediately known, British security officials said the threat appeared significant and involved Islamic extremists. 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."
Six men were arrested overnight and were being held under anti-terror legislation. A woman was also arrested Monday morning for failing to disclose information. The suspects are aged between 22 and 32. All were described as being British residents, and police would not give any details about nationalities or ethnicity.
The threat level remained at "substantial" Monday.
"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.
Britain lowered its assessment of a terrorist attack from "severe" to "substantial" in July. The current threat level means risk of an attack is a "strong possibility."
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.
The threat level was last at critical in June 2007, following an attack on Glasgow Airport and failed car bombings in central London.
Last year, British authorities arrested 12 suspects in a large-scale terror attack aimed at attacking British landmarks and public spaces.
At least five of the men were of Bangladeshi origin. Some of their cases are still pending.
Europe has been the target of numerous terror plots by Islamist militants.
The deadliest was the 2004 Madrid train bombings, when shrapnel-filled bombs exploded, killing 191 people and wounding about 1,800.
In 2005, suicide bombers killed 52 rush-hour commuters in London aboard three subway trains and a bus. In 2006, U.S. and British intelligence officials thwarted one of the largest plots yet, a plan to explode nearly a dozen trans-Atlantic airliners.