 Factors such as the massive state-sponsored migration of Han Chinese from the 1950s to the 1970s, government policies promoting Chinese cultural unity and punishing certain expressions of Uyghur identity, and heavy-handed responses to separatist terrorism have contributed to tension between Uyghurs, and state police and Han Chinese.This has taken the form of both frequent terrorist attacks and wider public unrest (such as the July 2009 Ürümqi riots).In recent years, government policy has been marked by mass surveillance, increased arrests, and a system of "re-education camps", estimated to hold hundreds of thousands of Uyghurs and members of other Muslim minority groups.