Police fired on hundreds of protesters in a Tibetan area of western China, killing eight people, overseas activist groups said. State media reported one government official was seriously injured in what it called a riot.

Another Tibetan activist group said on Saturday that two monks committed suicide late last month because of government oppression.

The reports indicate that unrest is continuing in China’s Tibetan areas despite a massive security presence in place since anti-government demonstrations in Tibet’s capital, Lhasa, and neighboring provinces broke out in mid-March.

The protests are the longest and most sustained challenge to China’s 57-year rule in the Himalayan region. China’s subsequent crackdown has drawn international scrutiny and criticism in the run-up to this summer’s Olympic Games.

Police fired on Buddhist monks and ordinary citizens who had marched on local government offices in Garze Tibetan Autonomous Prefecture in Sichuan province near Tibet on Thursday, according to the London-based Free Tibet Campaign and the International Campaign for Tibet.

The protesters were demanding the release of two monks who were detained after 3,000 paramilitary troops searched their monastery and found photographs of the Dalai Lama, Tibet’s exiled Buddhist leader, the groups said.

The US government-funded Radio Free Asia said it had unconfirmed reports that up to 15 people were killed and dozens injured in the violence. Calls to local police and hospitals in the area were not answered Saturday or else officials said they had no information.

The official Xinhua News Agency had no details on deaths or injuries but confirmed that a riot broke out in Donggu town in Garze.