By Maggie Fick

CAIRO, July 18 (Reuters) - When Mohamed Wahdan of the Muslim Brotherhood was arrested last week, he got a taste of what life may be like for the Islamist movement now that the army has overthrown President Mohamed Mursi. It was a familiar feeling.

Held for two nights with 24 other men in a packed cell 10 feet (three metres) square, Wahdan said the treatment was a chilling reminder of the oppression that the Brothers suffered during decades when Egypt was ruled by hostile military men.

"We couldn't sit. We couldn't pray, we couldn't sleep," he told Reuters. "This is the way of life we are greeted with after the coup," he said, referring to the army's takeover two weeks ago that Western governments have not yet termed as such.

When protests brought down veteran military autocrat Hosni Mubarak in 2011, the Brotherhood, which spent 85 years in the shadows as a secret society, burst into daylight as the dominant political force in the country, winning election after election.

But now that those victories have been reversed by another general, the Brothers are facing a future back underground.

"They are obsessed by their ordeal in the past," said Khalil al-Anani, an expert on Islamist movements at Durham University.

"They are right to be very concerned about their personal freedoms and their future."

The Brotherhood is unlikely, however, to return to the strategy of violence it abandoned decades ago, Anani said.

"The leadership realises it cannot sacrifice their image and credibility by using violence," said Anani. "They realise it is a useless tool in the political context, and they know the state will win any violent contest."

Brotherhood figures say the military is bent on driving them out of politics altogether, their leadership hauled off to jail.

They are staging a vigil at a Cairo mosque, now in its third week, with thousands of supporters vowing not to leave unless Mursi is restored in power. Every few days the movement calls mass demonstrations attracting tens of thousands of people, some of which have led to deadly clashes.

The confrontation only seems to be attracting a firmer crackdown from the authorities. Hundreds of Mursi followers have been rounded up, and arrest warrants have been issued for most of the Brotherhood's leaders.

"They remain in the streets protesting to ensure that the police state does not come back, which unfortunately seems to be happening now," said Anani.

"COMFORT ZONE"

The Brotherhood had already long since renounced violence when a more radical Islamist splinter group assassinated Mubarak's predecessor Anwar Sadat in 1979. Other radicals later broke away and continued violent attacks.

The Brotherhood, still banned, grew into an underground social welfare organisation, building up its numbers by preaching and charity, while members faced prison and torture.

Wahdan, a 52-year-old agriculture professor and a member of the Brotherhood's 17-man executive, spent three years in prison in the mid-1990s, convicted by a military court for involvement.

He said he now fears a return of the "police state".

"This started in the first hours after the coup," Wahdan said. "They closed our TV channels, they arrested our leaders and our activists, then they killed us and are torturing us in detention. The police cannot treat us this horrible way in 2013."

He was one of more than 600 Mursi supporters detained on July 8 after a clash in which more than 50 Mursi supporters and four members of the security forces were killed.

The authorities say the Brotherhood provoked that violence by attacking its soldiers; the Brotherhood says its partisans were peacefully praying. However the incident began, video footage showed snipers in uniform firing from rooftops at the crowd. Rights groups say troops used unnecessary force.

The authorities deny using disproportionate force or mistreating prisoners.

Coming into open politics was no easy decision for the Brotherhood. Spokesman Gehad el-Haddad said most of its older leadership opposed fielding a candidate for president. They were lobbied by younger activists who narrowly won an internal vote.

Haddad, 31 and a leader of the younger generation, lamented that the group's strides to adapt to overt politics could be reversed by authorities he said were intent on preventing it winning elections again, predicting more oppression to come.

"They need to dismantle the Brotherhood. They need to give the Brotherhood deep enough blows that it won't be able to contest anything new. How do they do that? Freezing the assets, arresting the top leaders, closing down the party and the Brotherhood headquarters and offices across the country, and killing people on the street. And they think we are going to budge?" he said at the group's vigil.

"This is an organisation built for 86 years under oppressive regimes. That is the nature of the organisation, that is our comfort zone. They just pushed us back into it."

Brotherhood activists are particularly frustrated that so many former allies from the uprising that ousted Mubarak now appear unmoved by the crackdown it is facing.

"We've seen a return of the state security officers, with the same faces, that we witnessed torturing Egyptians before the revolution," Brotherhood spokesman Ahmed Aref said at the Rabaa Adaweya mosque, site of the vigil.

A rallying cry in 2011 was the desire to end harsh police measures against Islamists and liberals alike. Although most of those Egyptians who do not support the Brotherhood are now firmly behind the military crackdown, a minority of liberals have begun expressing reservations.

But Haddad said he had stopped speaking to many liberal activists that he had long regarded as close friends.

LIFE AFTER THE "COUP"

The latest deadly violence took place in central Cairo late on Monday night. Witnesses said fighting broke out when Mursi supporters blocked traffic on a bridge over the Nile, and that police fired tear gas after the scuffles.

Pro-Mursi protesters said they were again attacked while praying, and that police intervened to support the attackers.

"Where were the ambulances?...How long will this go on? What is happening is a massacre," said a doctor who treated protesters. Seven people were killed and more than 260 wounded there and in another part of Cairo. More than 400 were detained.

Mursi himself is being held incommunicado at an undisclosed location with other leaders, including Haddad's father. Authorities say they are investigating Mursi over complaints of inciting violence, spying and wrecking the economy but have yet to charge him with a crime. A military spokesman said on Wednesday he was being held for his own protection.

Other senior Brotherhood figures are still camping out at the vigil, which turns every night into a carnival, attracting huge crowds, with vendors selling sweets and souvenirs.

Most of the leaders face arrest warrants, issued by the public prosecutor after Mursi's ouster, on charges such as inciting and funding violence or "thuggery".

Many of the older people at the camp were jailed under Mubarak. Some of the younger ones have family members who spent years in prison. On a recent, boiling afternoon, a man with a microphone called on them to pray during the daily Ramadan fast.

"This is a prayer in the heat, in the fasting, in the name of achieving the freedom of our president, legitimacy, justice," he said.

(Additional reporting by Peter Graff, Edmund Blair and Omar Fahmy; Writing by Maggie Fick and Peter Graff; Editing by Philippa Fletcher)