CAIRO, Jan 25 (Reuters) - Hundreds of youths clashed with Egyptian police in Tahrir Square on Friday in a violent start to the second anniversary of the uprising that toppled Hosni Mubarak and led to the election of an Islamist president who is now the focus of protester rage. Opponents of President Mohamed Mursi and his Muslim Brotherhood allies are expected to mass in Tahrir Square later on Friday 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 who threw petrol bombs and firecrackers as they tried to approach a wall blocking access to government buildings near the square. Plumes of teargas fired by the police filled the air. The Health Ministry said 16 people had been wounded. At one point, riot police used one of the incendiaries thrown at them to set ablaze at least two tents erected by the youths, a Reuters witness said.