Portugal News Online said 32 others were injured in the accident, which took place around 8 a.m. in Serta.
A spokesperson for the rescue team said all the victims were Portuguese, and the bus was heading toward Santa Maria de Feira for an excursion.
Euronews said the bus left the road and plunged down a ravine, likely due to the road's wet conditions.
More than 250 firefighters were on hand for the rescue operation, with one air ambulance helicopter also on the scene, Euronews reported.
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