BEIJING, July 1 (Reuters) - Last weekend's riot in southwest China over allegations of a cover-up in a girl's death was whipped up by people with ulterior motives and "gangsters", state media said on Tuesday.

About 30,000 people took to the streets -- some mobbing government offices and setting alight police vehicles -- in Weng'an county in Guizhou province, demanding justice over the teenager's death.

"The incident started from a simple cause but was used and incited by very few people with ulterior motives, and some gangsters took part," Xinhua news agency paraphrased provincial Party boss Shi Zongyuan as saying.

"We must put maintaining social harmony and stability on the top of our agenda," he said, adding minor offenders would be only cautioned.

Xinhua cited unnamed police sources as saying "people who had no knowledge about the details of the girl's death had been incited to violence". It did not elaborate.

Police, who have already said they will reopen the investigation into the girl's death, have been broadcasting appeals for calm and demanding people who "committed offences" turn themselves in, the report added.

The government has also started a nationwide stability push ahead of next month's Beijing Olympics to prevent any domestic unrest upsetting the Games and to present China as a harmonious nation.