Police said the bus plunged onto a van below the road early on Monday.
Local media said most of those who died were on the bus.
It was not clear what caused the crash but local reports cited rain, heavy traffic and speed as possible causes.
"We saw the bus fall from there and it flew, and then it hit the van," said shop owner Edith Voluntate. "When we got close, we just saw a lot of people pinned under it."