"The inside of the building is totally destroyed," a firefighter told the official Xinhua news agency after the blaze at the gothic-style Jiangbei Cathedral in Ningbo, in Zhejiang province.
"It's a great loss. The church is such a beautiful building."
No casualties were reported, the report said, and an investigation was under way.
It comes after authorities in April demolished a church in a heavily Christian area of Wenzhou, also in Zhejiang, in eastern China, after they said it was an illegal structure.
The Ningbo cathedral, constructed by a French bishop in 1872, is an important cultural relic protected by the state "for its great historical and architectural value", Xinhua said.
The agency quoted a member of the church's staff as saying that the fire began at the altar, although the man, who was staying in the building, did not know how it began.
The destruction of the Wenzhou church was seen as highlighting tensions between a rapidly growing number of Christian worshippers and China's officially atheist Communist Party-run government.
The ruling party keeps a tight grip on religion for fear it could challenge its grip on power, requiring followers to worship in places approved by the state and under government supervision.
The state-linked China Christian Council estimates the country has around 20 million Christians - not including Catholics - 70 percent of them living in rural areas.
The actual number, however, could be at least 40 million to 60 million, according to some estimates.
Overseas-based campaign group China Aid said last week that 14 members of a Wenzhou-based church were injured Monday morning when government officials stormed its gates to try and destroy its cross.
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