Seven people were killed on Wednesday when 15 letter bombs exploded in southern China, state media said, with blasts reported in multiple locations including government offices.

Police described the blasts, which injured 51 people on the eve of China's national day, as a "criminal case", ruling out a "terrorist act".

They said a 33-year-old local suspect had been arrested but gave no immediate explanations regarding his motive.

The explosions occurred in at least 13 locations in the rural county of Liucheng in the Guangxi region, the Nanguo Morning News, a local newspaper, cited police as saying.

They included a prison, a government office, a train station, a hospital and a shopping centre, it said.

Pictures showed portions of six-storey buildings gutted and collapsed, and streets littered with glass, bricks and other debris.

Other photos posted online, which could not be verified, showed overturned cars, victims bandaged and laid on makeshift stretchers and plumes of grey smoke rising above a residential district.

The explosives were apparently placed in express delivery packages, the official Xinhua news agency said.

"The public security department has quickly started to work and has already determined it was a criminal case," it quoted Liucheng police as saying.

Police arrested a "preliminary suspect", who was named as M. Wei, aged 33, and who lived in the town of Dapu in the county.

Liucheng county is under the administration of the city of Liuzhou.

-'Very scary' -

One witness told the South China Morning Post he was sitting in his shop when one of the blasts occurred.

"Some windows in my shop broke. I walked outside to see what had happened and was almost hit by a falling window from the third floor," said Li Acheng, 30.

"I saw half of a building nearby collapse.

"We were all very shocked and thought it might be an act of terrorism. All shops were closed and the town is under curfew with police guarding every street... It was very scary with so many attacks in just over an hour."

Another witness, who had gone to fetch his daughter from school recounted how he saw a scooter explode in front of the neighbouring hospital.

The father, carrying his crying child in his arms, then witnessed two more explosions in front of the gynaecological and disease control centres.

"We saw a passerby who had been hurt in the arm, moaning on the pavement," he told Chinese news portal Sina.

Since the blasts, about 60 "suspicious" packages have been reported to the authorities and are being examined, according to Liucheng police, highlighting the general anxiety sparked by the letter bombs.

The explosions took place on the eve of the national day holiday, during which some government offices and companies take the week off.

In recent years several disgruntled Chinese citizens have bombed local government offices and public places to try to draw attention to their grievances.

In 2013 a man set off a series of home-made bombs packed with ball-bearings outside a provincial government headquarters in northern China, killing at least one person and wounding eight.

Xinhua said at the time he sought to "take revenge on society".

The same year a street vendor set fire to a bus in east China's Fujian province, killing himself and nearly four dozen passengers in an act of retaliation against local authorities.

Legal paths for pursuing justice in China are limited, as courts are subject to political influence and corruption. Citizens who lodge complaints against authorities often find themselves being detained.

Authorities maintain tight control over public security in the one-party state and place huge importance on maintaining social order.

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Sina