The brawl between Han Chinese and Uighurs in southern China was scarcely covered by state media, but accounts and photos spread quickly via the Internet and became a spark that helped ignite deadly riots thousands of miles away in the Uighur homeland. Even in tightly controlled China, relatively unfettered commentaries and images circulating on Web sites helped stir up tensions and rally people to join an initially peaceful protest in the Xinjiang region that spiraled into violence last Sunday, leaving more than 150 people dead.