Diario de Noticias newspaper reported on Sunday that the police and Portuguese secret services said that the man had received training in a terrorist camp in Syria at the start of this year.
It said the detained individual, born in Angola but holding a Dutch passport, had a knife on him when detained on the tarmac last Thursday.
He had earlier been expelled from Turkey and was on authorities' watch lists in various European countries.
He was questioned by a judge on Saturday, who ordered that the man remain in custody due to the risk of him fleeing, SIC television reported.
Police officials were not immediately available to comment.
The arrest happened after the US government last week pushed for increased security precautions at European airports.
Security authorities are concerned that Al-Qaeda operatives in Syria and Yemen could try to blow up US or Europe-bound planes by concealing bombs.
NTJ/MB