Nine men have been arrested in Britain in a move police said was part of an ongoing operation against "Islamist-related terrorism" rather than in response to any immediate threat.
The men are held on suspicion of belonging to and supporting an organisation banned under anti-terrorism laws and "encouraging terrorism", police said.
The nine suspects, aged between 22 to 51, were taken to central London police stations while 19 properties across the capital and in Stoke-on-Trent in central England were being searched.
Police also raided homes, businesses and community centres on Thursday as part of the investigation.
Britain raised its international threat level to the second-highest level of "severe" last month, meaning that an attack is considered highly likely.
Prime Minister David Cameron has said the Islamic State of Iraq and the Levant group, which has recently taken control of territory in Syria and Iraq, poses the greatest-ever security risk to the country.