A British plane bound for Lisbon made an emergency landing in Oporto after crew found a suspicious package on board.
The airline operators ANA said that Portuguese airports were on maximum alert, but declined to give further details.
"Following the tragic events in the United States which shocked the world, Portuguese airports are on a maximum security alert," a statement from ANA said.
Television channel SIC Noticiassaid the flight from Gatwick airport had been chartered by GB Airways, and passengers had been evacuated.
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Police were also reported to be checking the plane's cargo for explosive devices and to have isolated one of the airport's runways.