By Alexander Dziadosz

CAIRO, June 30 (Reuters) - A few blocks from Tahrir Square, the eery silence of a shuttered downtown Cairo suddenly erupts into a roar of horns, drums, whistles and chants.

The smell of popcorn, cigarette smoke, grilled meat and freshly-squeezed oranges fills the air, as hundreds of thousands of protesters clap, cheer and wave the red-white-and-black Egyptian flag.

After months of calm, Tahrir, the festive heart of the 2011 uprising that toppled Hosni Mubarak, has burst back into life.

But in a reminder of the darker side of that protest encampment, women's activists said more than 30 women suffered organised sexual assaults by gangs of men in the square during Sunday's rally.

This time the target of the protesters' rancour was their first freely-elected president, Mohamed Mursi, accused of letting the country slide into anarchy and economic stagnation while he and his Muslim Brotherhood focus on monopolising power.

Fireworks exploded in purple, red and white bursts while green laser beams shot through the smoke. Military helicopters, a reminder of the armed force's central role, buzzed overhead.

"This is a revolution!" one young man shouted, pushing his way through packed sidestreets, where protesters perched atop juice shops and travel agencies and on the ornate wrought iron balconies of colonial-era buildings.

Families walked past walls festooned with layer upon layer of graffiti and campaign posters, testament to two-and-a-half years of non-stop political ferment.

In the square itself, the heat and pressure of bodies packed shoulder to shoulder in 38 Celsius (100 Fahrenheit) temperatures becomes sweltering. Plastic bags, soda bottles and orange peels cover the ground, which becomes wet and muddy in some places.

Men with hard hats reading "Tahrir Bodyguard" and green reflective vests patrol the crowd. Young men wearing headbands reading "Rebel" and red cards reading "Get out!" Children with Egyptian flags painted on their cheeks are hoisted on shoulders.

Sunday's protests were the biggest since the revolt against Mubarak, and Tahrir's festive atmosphere was largely reminiscent of those days - except for the obvious absence of Islamists, who have staged a separate rally to support the president.

Protesters recycled chants from the 2011 uprising: "The people want the downfall of the regime," and "We won't leave - he will leave". Whether they are able to sustain the occupation of the central Cairo square until they achieve their objective this time remains to be seen.

Egyptians also took to Tahrir in large numbers after Mursi was elected to celebrate his narrow victory over a Mubarak-era military officer whom many feared would revive the police state they had only just cast off.

But a widespread perception that the Islamist-led government has failed to restore stability and economic prosperity has turned many against Mursi since then.

"Look at the country. It's gone back 20 years," said Ahmed Ali el-Badry, an animal feed salesman wearing a white robe.

"We want a just leader who brings us freedom and social justice. It doesn't matter who he is but he has to be just."

Asked if he was worried that Egypt would start overthrowing its president every year, he said that was impossible: "If we get a just leader why would we oust him?" (Additional reporting by Shaimaa Fayed, Patrick Werr and Yasmine Saleh; Editing by Paul Taylor)