Twelve people were injured Sunday after decorative balloons filled with hydrogen exploded at a ceremony at a university in northwest China's Shaanxi Province, local government and hospital sources said Monday.

The injured are treated in hospital.

Several balloons blew up at around 10 a.m. as about 20,000 students, teachers and alumni gathered at Weinan Teachers University to celebrate the 50th anniversary of the university's founding, a spokesman with the Weinan city government said.

A witness surnamed Sun said he saw the balloons blow up and catch fire mid-air, injuring at least a dozen people on the face, neck and arms.

The 12 injured were treated at nearby Honghua Hospital. Eight were hospitalized while four were allowed to return home, a hospital spokesman said.

The accident is under investigation.

Weinan Teachers University was founded in 1960 in Weinan, a city with about 3 million people east of the provincial capital, Xi'an.