Hundreds of sunbathers were on the popular São João da Caparica beach, 20 miles (30km) south of Lisbon, at the time. Officials said they were not aware of any other injuries.
“A plane with two people on board made an emergency landing on São João da Caparica beach, striking two sunbathers and causing their deaths,” Portugal’s national maritime authority said in a statement.
The two occupants of the plane were unhurt and were being questioned by officers from Portugal’s national maritime police. The small aircraft, a Cessna, appeared to be in difficulty and wobbled as it skimmed low over the sunbathers, scattering them and spreading panic on the beach, witnesses said.
The plane appeared to have a partly broken left wing, which hung lower than the right wing, according to Portuguese TV pictures. Local reports said the aircraft was registered to a flying school in Torres Vedras.
The Portuguese newspaper Diário de Notícias reported that the Aerocondor aviation school had confirmed the aircraft was “on a training flight with one student and one senior instructor”, who had “high experience and thousands of hours of flying time”.
The dead girl was with her parents, who were unhurt, witnesses told local media. The plane passed over the dead man’s legs as he sunbathed on a towel, they said.
One witness, Rita Rogado, 20, told the Público newspaper that the plane crashed right by the sea, dragging her along for a few metres in the sand.
Some sunbathers said the plane appeared to be gliding, as they heard no engine noise.