LONDON Police arrested a 24-year-old man at 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 October 16. The woman was released without charge.