CAIRO (Reuters) -- Egypt's revolutionaries did not take to the streets to replace Hosni Mubarak with another military strongman or to put an Islamist ideologue in charge, but that is the choice they woke up to after a first-round vote for the presidency.

The youths who put national pride before religion when they protested against Mubarak's autocratic rule last year have increasingly despaired, saying the revolution they initiated has been hijacked by generals and the Muslim Brotherhood.

Their worst fears were confirmed on Friday, when initial results from Egypt's first free presidential election sent the Brotherhood's Mohamed Mursi and ex-air force chief Ahmed Shafiq, Mubarak's last prime minister, into a June 16 and 17 run-off.

"I am in shock. How could this happen? The people don't want Mursi or Shafiq. This is a catastrophe for all of us," said Tareq Farouq, 34, a Cairo driver. "They are driving people back to Tahrir Square."

With moderate candidates now out of the 12-man race, the run-off pits the two most polarizing figures against each other, reviving the decades-old power struggle between Egypt's secular-led military elite and its powerful Islamist opposition.

The protesters of Tahrir Square are shocked that the run-off has boiled down to a member of the "feloul", the derisory Arabic term for "remnants" of Mubarak's old guard, and an "Ikhwani", or a Brother, from the conservative Islamist group that has battled the authorities for most of its 84-year-old history.

"Ahmed Shafiq will mean the old regime - the revolution is liquidated - and with the Muslim Brotherhood it means we are too near to some kind of religious state," said Hassan Nafaa, a political scientist who sided with the street against Mubarak.

For revolutionaries, Shafiq is a carbon copy of Mubarak. Both were air force commanders and a Shafiq win would simply extend the 60-year tradition of having military men at the helm.

But the Brotherhood, which has the biggest parliamentary bloc, is just as unattractive for many of them, with its pledge to apply Islamic sharia law that they fear will curb social freedoms, stifle liberal debate and squeeze out other voices.

"What happened to our revolution? A Shafiq victory means a reproduction of the old regime and a Mursi victory will be a disaster. The Brotherhood will be in control of the presidency and parliament and will have a monopoly over everything," said Mohamed Hanafi, 30, a factory worker.

"We don't know where it will all end," he said.

It is not just the fate of Egypt's 82 million people. What happens in the Arab world's most populous nation will reverberate across a region convulsed by revolts and conflicts.

An Islamist takeover could frighten liberal forces which have played a big part in the Arab uprisings that toppled leaders in Tunisia, Libya and Yemen, as well as Egypt.

The return of a military strongman could embolden Syria's leadership which has used tanks to try to crush a rebellion.

Yet, 15 months after hundreds of thousands of people packed Tahrir Square to celebrate the end of Mubarak's three-decade rule, exhilaration has given way to disillusion and anger.

The transition, overseen by generals in charge since Mubarak fell and who have promised to hand over power by July 1, has been marred by violence, political gridlock and economic woes.

The second round vote may bring more turbulence. Activists and other Egyptians have already said they will go back to the streets if Shafiq is victorious, though this may be more difficult to sustain if the second round proceeds smoothly.

Whoever wins, the army is likely to remain the main power broker, giving it the upper hand in defining the prerogatives of the president. These have yet to be determined because of tussles over who should write the post-Mubarak constitution.

The generals insist they will return quietly to barracks, but most Egyptians do not expect them to give up the privileges and influence they have enjoyed for decades. They say a Shafiq win will make rolling back the military harder still.

Whatever the president's powers, the army will keep a tight grip on foreign policy and will protect a peace treaty with Israel that brings in $1.3 billion of US military aid a year.

This may restrict a Mursi presidency's room to maneuver abroad, but put a bigger focus on work at home. Liberals and some other Egyptians fear the Islamists will want to stamp their mark by seeking to impose more religious values on society.

"This is not what we wanted or what we fought for. As a woman I am deeply upset," said Dalia Hamdi, an human resources manager, speaking in the upscale Cairo district of Zamalek.

"Definitely we won't be able to wear what we are wearing now," she said pointing to a friend in a sleeveless t-shirt and jeans without a headscarf, which most Egyptian women wear.

Mursi has not defined what his call for implementing sharia law would mean in practice. But liberals fear it could mean more restrictive marriage and divorce laws for women. A stricter society might also deter tourists, a main revenue source.

Near the Giza pyramids, on a street famed for its night life, some bar workers said they dreaded a Brotherhood president, recalling how an Islamist mob vandalized nearby casinos and bars last year, smashing liquor bottles seen as symbols of vice.

"I am sure the Brotherhood will close us down if they get elected. They will ban alcohol," said bar-tender Huda Husseini, 19. "According to them, dancing, drinking, music is all forbidden. Everything is forbidden."