Xiamen police did not provide further details in making the announcement online.
The fire ripped through the bus during the Friday evening commute while it traveled on an enclosed and elevated road in the city. A local fire official said emergency responders found bodies piled inside the charred, skeletal bus. The official Xinhua News Agency said Saturday that in addition to the 47 dead, 34 people remained hospitalized.
Investigators said early Saturday that the fire appeared to be intentionally set, and not an accident, Xinhua reported. Among the clues, Xinhua said, was that investigators found traces of gasoline on the wreckage, though the bus ran on diesel fuel and its oil tank and tires remained intact.
"It's a serious criminal case," the Ministry of Public Security, the national police agency, said in a statement on its website. Xiamen police declined further comment.
China has seen bombings and arson of buses and public buildings in recent years, sometimes by people trying to settle personal scores and also by people with overtly political grievances. In 2009, an unemployed man set fire to a packed bus in the central city of Chengdu, killing himself and 26 others.
Xiamen, a prosperous trading port known for centuries in the West as Amoy, was rattled by Friday's fire. The city immediately suspended service of the entire express bus system, known as Bus Rapid Transit, but operations resumed Saturday morning.
Witnesses quoted by Xiamen's official news site said they heard sounds of explosions after the fire had been burning for 10 minutes. Photos posed online showed heavy black smoke curling from the burning bus on the elevated road.