Dusk replaced the day's white heat with a tepid glow and Cairo's din faded into an expectant silence as Amu Faud sat poised over his street-side dinner table, awaiting the chants from a dozen mosques that would signal that the sun had finally set.

Across the city millions of Egyptians were sitting like Amu Faud, at the ready for their food and drink since the dawn 15 hours earlier. Bureaucrats' families gathered around apartment dining tables. Policemen sat around tin plates brought by their wives to street corners. The rich poured fruit juice to ready their shrunken stomachs for an all-night feast while the poor prepared beans as they squatted in the dust.

When the cannon boomed and muezzineen cried, "God is great" from their minarets, the entire city seemed to bend its head toward a plate. Normally clogged streets were empty. Telephone lines suddenly were free.

Shops and offices had cleared and the calm that settled on this noisy city of 10 million was so complete that, after the first gulps had gone down, table talk was clearly audible in the streets where normally pedestrians have to shout to make themselves heard.

Ramadan, the sacred Moslem month of fasting and prayer now in its second week, has taken over Cairo and the Middle East. The holy period -- the ninth month of the lunar calendar, when the angel Gabriel revealed the Koran to Mohammed in a cave near Mecca -- is an annual feast reminiscent of Christmas and New Year's in the Christian world.

But it lasts an entire month and with its obligatory total fast, brings much of the region's economic and official activity to a near halt. Islamic law says all able Moslems must abstain from food, drink, sex, tobacco and evil from dawn to sunset during Ramadan.

The original idea was mortification of the flesh and exaltation of the soul.

But the modern result is that many Moslems stay up most of the night eating and drinking, checking into the office for a few hours in the late morning and sleeping away the afternoon until, like Amu Faud beside his soft-drink stand, they can eat again in good conscience.

The fasting is particularly difficult this year, however, because Ramadan falls early. The heat is severe in July -- in the nineties these days in Cairo, accompanied by unusual humidity. But worse, the spread between dawn and dusk is longer than it has been for a number of years, making the fast longer than ever.

"I was fasting, but I had to give up after eight days," a young Cairo woman complained. "I was falling asleep every 20 minutes.

The Moslem lunar calender backs up about nine days every year with respect to the Roman calender used in the West. The fasting thus has moved in recent years from the relative cool of fall, with its shorter days, to the heat of summer, with its long daylight periods. Next year, Moslems predict, the fast will be even harder to bear since it will be still longer.

But mortification sets with the sun and Ramadan is also a period of feasting and revelry. This is particularly true in Cairo, where Egyptians turn the breaking of the fast into an all-night party. As a result, many Moslems from more austere Arab countries such as Saudi Arabia and Kuwait pick Ramadan as the time to visit. Their white robes and checkered headdress are seen coming and going at Cairo's nightclubs more often than at its mosques.

The season does, however, include increased piety. The Egyptian government has expanded the air time alloted to Koranic readings on television and set up a gaudily lighted tent near the Abideen presidential headquarters where famous sheiks preach nightly sermons.

Two brothers who own a successful antique business, Ali Ahmed and Hassan Ahmed, feed between 150 and 200 poor people every evening at a twilight breakfast on the sidewalk in front of their store. The gesture is a long Ramadan tradition in the family.

"We do it because of Allah," Ali Ahmed told a visitor recently as about 80 men sat eating at a table and their women and children ate on the ground beside them.

Later in the evening about 100 men sat sipping free tea and listening to a sermon from Sheik Ahmed Badri Abul Aila in the Abideen tent. Women also were allowed to listen but in a special section reserved for them and separated from the main tent by waist-high barriers. The tea never made it to their little pen.

The Abideen congregation seemed tiny, however, compared to the thousands of Egyptians milling around the cafes and restaurants of Khan Al Khalili, Cairo's old market quarter next to the prestigious Hussein Mosque. Laila, a tired ballad singer rattled her tambourine and begged for tips from customers at Fishawi, a vulnerable Cairo coffee house, while rows of devout men bobbed and weaved in a ritual dance in front of the mosque.

The rooftop restaurant atop the neighboring Hussein Hotel was packed with Egyptians. As they ate and drank, they looked down on the masses pushing through the market's narrow alleys and through the broad mosque square where Mamdouh Sadhiq was offering his charcoal portraits cheap to passers-by.

The strolls through Khan Al Khalili and dozens of other Cairo neighborhoods decorated with lights and sweets stalls are what Ramadan has come to mean for most Cairo residents. The carnival atmosphere lasts until before dawn, when they must eat again to store away nourishment for the daylight fasting hours.

Their special Ramadan meals often open with qamar al-din, a juice made from dried apricots boiled in water and then chilled. Following that, tradition calls for some tea and, once the stomach has stretched back into shape with the liquid, a feast of sweets and meat.

Some Egyptians say all-day fasting often leaves them without appetite and jokes are circulating about bountiful breakfast dinners untouched by husbands who only felt like drinking after a day of thirst. Loss of appetite is more talked about than suffered from, however, and the Ministry of Supplies estimate that consumption of sugar, flour and meat rises sharply during Ramadan. Shortly before the season began this year, the ministry announced that rations in Cairo were being doubled for the month.