Former Guantanamo Bay detainee Moazzam Begg is among four people who have been arrested on suspicion of Syria-related terrorism offences, police have said.
Mr Begg, 45, who was held by the US authorities for almost three years, was arrested earlier with two other men and a woman in the West Midlands.
He was detained on suspicion of attending a terrorist training camp and facilitating terrorism overseas.
West Midlands police said naming Mr Begg did "not imply any guilt".
A spokeswoman added: "We can confirm that Moazzam Begg was arrested this morning. We are confirming this name as a result of the anticipated high public interest."
Mr Begg was arrested in Hall Green in Birmingham. The other people arrested - a 36-year-old man from Shirley, a 44-year-old woman and her son, 20, both from Sparkhill - are being held on suspicion of facilitating terrorism overseas.
Mr Begg, a British citizen originally from Birmingham, moved to Afghanistan with his family in 2001, before moving to Pakistan in 2002 when the Afghanistan war started.
He was arrested in Islamabad in January 2002 and taken to Bagram internment centre in Afghanistan for about a year before being transferred to the Guantanamo Bay US detention camp in Cuba.
He was released in January 2005 with three other British citizens and returned to the UK. Mr Begg, who is now a director of campaign group Cage which helps the families of people who have been detained, has never been charged with an offence.
He has always maintained that he was only involved in charity work and has never been involved in any kind of terrorist activity.
Our correspondent added that the arrests were part of a wider pattern and an increasing focus on Syria - there were 16 such arrests in January alone compared with 24 in the whole of last year.
Police say all four people arrested on Tuesday morning are being held at a police station in the West Midlands. Det Supt Shaun Edwards said the four arrests were connected.
He said: "They were pre-planned and intelligence-led. There was no immediate risk to public safety."
Counter-terrorism police are searching their three homes and vehicles and electronic equipment have been taken away for forensic analysis, West Midlands Police said.