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.
The square was calm by daybreak, following early morning battles between police and protesters.
But by mid-day, plumes of teargas fired by the police filled the air.
Egypt's Health Ministry said 16 people had been wounded in the clashes so far.
The anniversary will once again showcase the divide between Egypt's Islamists and their secular opponents.
Clashes mark Egypt revolt's anniversary (1:05)
Jan. 25 - Hundreds clash with police in Cairo on the second anniversary of the Egyptian uprising. Sarah Sheffer reports. ( Transcript )