Police arrested a 24-year-old man at London's Heathrow airport on Monday as part of an investigation into people travelling to Syria for "alleged terrorist activity".
The man, who had arrived in the UK on a plane from Bahrain, was held "on suspicion of the commission, preparation or instigation of acts of terrorism", said the force.
He was taken to a police station in south London for questioning by officers from Counter Terrorism Command, the police added in a statement.
Al Qaeda leader Ayman al-Zawahri last month called on Muslims to join the rebellion against Syrian President Bashar al-Assad, according to the SITE monitoring service.
Islamist militant groups have played an increasingly significant role in the fighting.
British police arrested a man and a woman at Heathrow last month as part of similar investigations into travel to Syria . A 26-year-old man was charged on Oct. 16. The woman was released without charge.