A grenade attack has killed 16 policemen and left 16 others injured at a border patrol station in north-west China's Xinjiang region, just four days before the opening of the Olympic games.
The attackers used a dump truck to ram their way into the police station and then tossed the two grenades.
Police have arrested the two attackers.
The official Xinhua news agency did not further identify the attackers, but the the Xinjiang area has long been home to a simmering rebellion by the local Musim Turkic people, the Uygur, who oppose Chinese rule.
Meanwhile a small group of protesters has clashed with police near Tiananmen Square in China's capital, saying the happiness of the Beijing Olympics was built on their pain.
The two dozen demonstrators scuffled with police today in the historic Qianmen district just south of Tiananmen.
The protesters said they had been evicted from their homes to make way for reconstruction of the district. The area is being rebuilt into a commercial strip with businesses such as Nike, Starbucks and Rolex.
Demonstrations in and near central Beijing's Tiananmen Square are rare and generally stopped quickly by police.
China is sensitive to any public criticism of the Olympics, and has stationed security agents throughout the city to watch for signs of unrest.