British former Guantanamo Bay detainee Moazzam Begg has been arrested on suspicion of Syria-related terrorism offences.
Mr Begg, 45, from Hall Green, in Birmingham, who was held in the US-run military prison in Cuba for nearly three years, is suspected of attending a terrorist training camp and facilitating terrorism overseas, West Midlands Police said.
A 36-year-old man from Shirley, in Solihull, and a 44-year-old woman and her 20-year-old son, both from Sparkhill, in Birmingham, have also been arrested on suspicion of facilitating terrorism overseas.
All four are being held at a police station in the West Midlands, police said, while their three home addresses are being searched by officers from the West Midlands Counter Terrorism Unit. Vehicles and electronic equipment are being removed for forensic analysis.
A West Midlands Police spokeswoman said: "We can confirm that Moazzam Begg was arrested this morning.
"We are confirming this name as a result of the anticipated high public interest to accredited media."
She added naming Mr Begg does "not imply any guilt".
Head of investigations for West Midlands Counter Terrorism Unit, Detective Superintendent Shaun Edwards, said: "All four arrests are connected. They were pre-planned and intelligence led. There was no immediate risk to public safety.
"We continue to urge anyone planning to travel to Syria to read the advice issued by the Foreign Office."
Originally from Birmingham, Mr Begg moved to Afghanistan with his family in 2001 before taking them to Pakistan in 2002 when the war began.
He was detained in Islamabad, Pakistan, as an " enemy combatant" in January 2002 and was taken to the Bagram internment centre for about a year before being transferred to Guantanamo.
The British citizen was released along with three others in January 2005 and was allowed to return to the UK where he was arrested by the police before being released without charge.
Mr Begg, now a director of campaign group Cage, has always maintained that he was only involved in charity business and that he has never been involved in any kind of terrorist activity.