The four were arrested at their homes in the southwest of Birmingham, Britain's second city, in central England, the West Midlands Police said.
A 45-year-old man was arrested on suspicion of attending a terror training camp and facilitating terror abroad.
A 44-year-old woman and her son, aged 20, were held on suspicion of facilitating terror abroad, as was a 36-year-old man at a separate address.
The three homes were being searched by officers from the West Midlands Counter Terrorism Unit, while vehicles and electronic equipment were being removed for forensic analysis.
"All four arrests are connected," said Detective Superintendent Shaun Edwards, the WM CTU head of investigations.

"They were pre-planned and intelligence-led,” he added.
The suspects, who were not named, were being held at a police station in the Birmingham area.


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