* Curfew enforced by soldiers and vigilantes
* Sinister atmosphere in biggest Arab city
* "Popular committees" claim to catch roaming Islamists
By Michael Georgy
CAIRO, Aug 18 (Reuters) - When night falls in Cairo, a security crackdown on the Muslim Brotherhood quickly turns the Arab world's most vibrant city into a ghost town run mostly by vigilantes eager to hunt down members of the Islamist group.
The sound of boats blaring music on the Nile and hawkers selling fruit juice and nuts fades as a dusk-to-dawn curfew takes hold after the bloodiest week in Cairo's modern history.
On Wednesday, security forces broke up protest camps set up by Brotherhood supporters to demand the return to power of the Islamist president, Mohamed Mursi, ousted by the army on July 3.
More than 700 people have been killed in the clampdown and subsequent clashes between security forces and the Brotherhood. A state of emergency was imposed, along with the curfew.
After most Egyptians have gone home at night, Cairo turns eerie as the military fans out, parking armoured personnel carriers on highways and beside bridges and installations.
When it comes to smaller roads and alleyways, the soldiers seem happy to let the vigilantes, better known as "Popular Committees", run the show, along with a few policemen in black hoods who line up suspects along bridges overlooking the Nile.
Some of the volunteers seem to be respected older men who command authority in dusty, run-down Cairo neighbourhoods.
Then there are youngsters like Kano, an energetic 16-year-old who stood at a street corner inspecting the few passing cars. "Long live Egypt," he yells, then laughs.
The Popular Committees say they have intercepted vehicles with weapons used by Muslim Brotherhood "terrorists", and tackle the thieves who seem to flourish in Egypt's chaos.
WHISTLING FOR SOLDIERS
In one neighbourhood, vigilantes took a break from manning their makeshift checkpoint and sipped tea. They explained how the system works. When someone suspicious is stopped, they use a whistle to alert nearby soldiers, or call the closest state intelligence agent on his cell phone. Or they act alone.
"When we catch someone, believe me, hundreds of us can quickly deploy to help each other if we can't reach the army," said Mohamed Shaaban.
A couple of nights ago, he said they had caught 47 Brotherhood members primed to attack an Interior Ministry building. An associate proudly displays a telephone video of the men being pushed and punched as they are loaded onto a police vehicle.
Another man showed a bullet that he said he had taken off one of the alleged Brotherhood activists. Then an angry woman loudly echoes what has become the official line: "They are dangerous people. They can invade your home. They are dogs."
Mursi became highly unpopular because he was seen as trying to monopolise power and failed to improve the economy.
Nearby, soldiers are more reserved, avoiding the subject of the Brotherhood but defending the military takeover, which followed mass protests against Mursi's rule.
"The media calls it a coup. But it was not a coup. We were responding to the will of the people," said one, as an officer sternly told his men to take up positions along a dark street.
Many Egyptians wonder where the turmoil will lead the nation of 85 million, which has lurched from one crisis to another since a revolt toppled autocrat Hosni Mubarak in 2011.
As the vigilantes kept up their mission and soldiers watched from a distance, a man who worked as an egg vendor before the latest trouble erupted shook his head as he watched the news.
"Life was difficult under Mubarak. Now it is unbearable. How can we survive when we can't even move around at night and sell our eggs? Is this a life?" (Editing by Alistair Lyon and Kevin Liffey)
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