Description:Egyptian police lob a tear gas canister over a security barrier as youth retaliate with rocks. It's a violent start to the second anniversary of Egypt's uprising that toppled Hosni Mubarak and ushered in an Islamist-led government. By afternoon prayer time, thousands of opponents of President Mohamed Mursi and his Muslim Brotherhood allies had gathered in Tahrir Square -- there to revive the demands of a revolution that they say has been betrayed by the Islamists.(1:04)