At least one person killed and more injured amid several blasts outside building in Taiyuan, capital of Shanxi province

At least one person was killed and eight injured in a series of explosions outside a provincial Communist party office in central China on Wednesday morning.

The blasts occurred at 7.40am outside of the Shanxi Communist party committee offices in Taiyuan city, the provincial capital of Shanxi, according to China's official newswire Xinhua.

"Judging by steel ball bearings scattered throughout the scene, [police] suspect an improvised explosive device," Xinhua reported. "Right now the police have blocked off the scene and the incident is under investigation."

The state broadcaster CCTV attributed the blasts to seven bombs hidden in roadside flower displays. More than 20 vehicles were damaged, CCTV reported. The motive for the attack was unclear.

"About 7.40 in the morning I heard some explosions," said a witness surnamed Gao, who lives in a dormitory close to the party offices. "At first I'd thought they were only firecrackers or something until I heard a police whistle. I realised something was wrong and immediately went downstairs to the scene.

"I saw blood stains on the ground, and an old lady whose head was injured by flying debris – she was being rescued and sent to the hospital. I also saw that the car windows in a nearby parking lot were shattered."

An elderly man described the scene in a video posted online. "I saw an iron ball flying from 300 to 400 metres away and hit an old lady on the head," he said. "Look, look at the blood stain on the ground. I saw seven blasts and three of them were continuous. The iron ball was as big as a walnut. The old lady was accompanying her grandson to school and got hit. Luckily the kid was fine."

Pictures posted online showed a column of smoke rising in front of the building near a congested thoroughfare; others showed one person sprawled in the middle of the road, which was lined with fire engines.

China has been on high alert for violent attacks since last week, when a sports utility vehicle ploughed through pedestrians in Tiananmen Square, crashed into a marble bridge and exploded, killing five people and injuring 40. Authorities labelled that incident a terrorist attack and blamed the East Turkestan Islamic Movement, a shadowy group based in Xinjiang, a north-western region of China.

In a handful of recent incidents, disgruntled Chinese citizens have taken violent measures to avenge perceived mistreatment by authorities. In July a wheelchair-bound man set off an explosive device at the Beijing Capital international airport to protest against official treatment of a decade-old case of police brutality.

In 2011 an unemployed farmer in Fuzhou, a city in eastern Jiangxi province, blew himself up near government buildings to protest at the confiscation of his land to build a highway. Three people were killed including the farmer, and seven more were injured.