Hundreds of youths clashed with Egyptian police in Tahrir Square on Friday in a violent start to the second anniversary of the unrest that toppled Hosni Mubarak and led to the election of an Islamist president.

Opponents of President Mohamed Morsi and his Muslim Brotherhood allies gathered in Tahrir Square on Friday to revive the demands of a revolution that they say has been betrayed by the Islamists.

Protesters against Egypt's President Mohamed Morsi light fireworks as they gather at Tahrir Square in Cairo on Thursday. [Photo/Agencies]

The square was calm following battles since Thursday night between police and protesters who threw gasoline bombs and firecrackers as they tried to approach a wall blocking access to government buildings near the square.

Plumes of tear gas 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.

"The people want to bring down the regime," declared banners in the square. "Save Egypt from the rule of the Supreme Guide," said another, a reference to the leader of the Muslim Brotherhood, Mohamed Badie.

Morsi, in a speech on Thursday marking the Prophet Mohammad's birthday, called on Egyptians to celebrate the anniversary "in a civilized, peaceful way that safeguards our nation, our institutions, our lives".

"The Brotherhood is very concerned about escalation, that's why they have tried to dial down their role on Jan 25," said Shadi Hamid, director of research at the Brookings Doha Center.

"There may very well be the kinds of clashes that we've seen before, but I don't see anything major happening that is going to fundamentally change the political situation," he said.

Morsi faces discontent on multiple fronts.

His opponents say he and his group are seeking to dominate the post-Mubarak order.

The Brotherhood dismisses such criticism as unfair. It accuses its opponents of failing to respect the rules of the new democracy that put the Islamists in the driving seat by winning elections.

Six months into office, Morsi is also being held responsible for an economic crisis caused by two years of turmoil. The Egyptian pound has sunk to record lows against the dollar.

The parties that have called for Friday's protest list demands including a complete overhaul of the constitution that was fast-tracked into law by Morsi in December, a move that fueled street violence.

Its critics say the constitution, which was approved in a popular referendum, offers inadequate protection for human rights, gives the president too many privileges and fails to curb the power of the military establishment.

Morsi's supporters say the criticism is unfair, that enacting the constitution quickly was crucial to restoring stability, and that the opposition is making the situation worse by perpetuating unrest.

With its eye firmly on forthcoming parliamentary elections, the Brotherhood is marking the anniversary with a big charity campaign. It aims to deliver medical aid to 1 million people, offer affordable basic foods, and renovate some 2,000 schools.