* Islamists' opponents to rally on uprising anniversary
* Teargas, petrol bombs fly as youths clash with police
* Brotherhood decides to stay away from Tahrir Square
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.
Inspired by Tunisia's uprising against President Zine al-Abidine Ben Ali, Egypt's revolution helped set off more revolts in Libya and Syria. But the sense of common purpose that united Egyptians at the time has given way to conflict that has grown only worse and last month triggered lethal street battles.
The anniversary will once again showcase the divide between the Islamists and their secular opponents. The Brotherhood has decided against mobilising in the street for the occasion, a decision that could reduce the likelihood of confrontation.
"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 leader of the Muslim Brotherhood, Mohamed Badie.
Mursi, in a speech on Thursday marking the Prophet Mohammad's birthday, called on Egyptians to mark the anniversary "in a civilised, 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.
Mursi faces discontent on multiple fronts.
His opponents say he and his group are seeking to dominate the post-Mubarak order. They accuse him of showing some of the autocratic impulses of the deposed leader by, for example, driving through a controversial new constitution last month.
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, Mursi 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.
Other sources of friction abound. Activists are impatient for justice for the victims of political violence perpetrated over the last two years. Little has been done to reform brutal Mubarak-era security agencies. A spate of transport disasters on roads and railways neglected for years is feeding discontent.
The parties that have called for Friday's protest list demands including a complete overhaul of the Islamist-tinged constitution that was fast-tracked into law by Mursi in December, a move that fuelled 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.
Mursi'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.
A strong turnout on Friday could also help the Brotherhood's opponents ahead of the elections. "There is a lot of power in this day and a real chance to use that to mobilise their supporters in the lead-up to the elections," Hamid said.