A bomb exploded in a McDonald's restaurant in the centre of Xian at the weekend and another fast food restaurant in the city received a threatening phone call yesterday, police said.
Nobody was killed or injured in the blast in Shaanxi province.
Investigators have classified the case as sabotage, and determined that the bomb was homemade, said a spokesman for the city's police, surnamed Jia.
He said no suspect had been identified and there had been no arrest over the explosion which happened at 3.50pm on Sunday in a toilet of the Zhonggulou restaurant, one of McDonald's two branches in Xian.
The Zhonggulou branch has only been open for five months.
The other McDonald's outlet in the city, in the Kaiyuan district, received an anonymous call threatening the safety of customers a day earlier, according to the China News Service. It was a hoax.
The Kaiyuan branch, which was the first McDonald's restaurant in west China, was hit by a bomb in December 2001 which killed two and injured 27, only four months after it opened.