A 33-year-old Portuguese citizen has been detained in the UK on suspicion of having connections with a terrorist group in the Middle East, a minister of foreign affairs source confirmed to Lusa news agency.
The British police also arrested three other men believed to be involved in planning terrorist acts in Syria.
Scotland Yard Metropolitan Police said the Portuguese man was detained at Gatwick airport as he was preparing to board a plane.
In the same investigation, three Britons, aged 18, 31 and 22, were arrested at separate addresses in east London on Wednesday 9 suspected of plans to travel to Syria in support of jihad terrorist activity. They are being held at a south London police station.
According to Diário de Notícias newspaper, the Portuguese is also being questioned about the abduction of two journalists in Syria last year.
The police operation follows the arrests of two men last year who are accused of being part of an extremist group which held John Cantlie, a photographer, and his Dutch colleague Jeroen Oerlemans in Syria. They were taken hostage on July 7 on the Syrian border with Turkey and released nine days later.
The two men, arrested last year, have already appeared in court. Shajul Islam, 26, a trainee NHS doctor, was arrested at Heathrow airport in October and one month later, Jubayer Chowdhury, 24, was arrested at Heathrow after arriving on a flight from Bahrain. They both face trial later this year.
The British intelligence services have previously warned that as many as 100 Britons may have travelled to Syria to join jihadist terrorist groups.
