Egypt is the Arab world's largest Sunni country, but as a writer once quipped, it has a Shiite heart and a Sunni mind.
In its eclectic popular culture, Sunnis enjoy a sweet dish with raisins and nuts to mark Ashura, the most sacred Shiite holiday. Raucous festivals bring Cairenes into the street to celebrate the birthdays of Shiite saints, a practice disparaged by austere Sunnis.
The city's religious quarter tangles like a vine around a shrine to Imam Hussain, one of Shiite Islam's most revered figures.
The syncretic blend makes the words of Mahmoud Ahmad, a book vendor sitting on the shrine's marble and granite promenade, even more striking.
"The Shiites are rising," he said, arching his eyebrows in an expression suggesting both revelation and fear.
The growing Sunni-Shiite divide is roiling an Arab world as unsettled as at any time in a generation.
Fought in speeches, newspaper columns, rumours swirling through cafes and the internet and occasional bursts of strife, the conflict is predominantly shaped by politics: a disintegrating Iraq, an ascendant Iran, a sense of Arab powerlessness and a persistent suspicion of American intentions. But the division has begun to seep into the region's social fabric, too.
The sectarian fault line has long existed and sometimes ruptured, but never, perhaps, has it been revealed in such a stark, disruptive fashion.
Newspapers are replete with assertions, some little more than incendiary rumours, of Shiite aggressiveness. The Jordanian newspaper Ad Dustour, aligned with the government, wrote of a conspiracy last month to spread Shiism from India to Egypt. On the conspirators' agenda, it said: assassinating "prominent Sunni figures".
The same day, an Algerian newspaper reported that parents were calling on the government to stop Shiite proselytising in schools. An Egyptian columnist accused Iran of trying to convert Sunnis to Shiism in an attempt to revive the Persian Safavid dynasty, which came to power in the 16th century.
At Madbuli's, a storeyed bookstore in downtown Cairo, five new titles lined the display window: The Shiites, The Shiites in History, Twelve Shiites, and so on. A newspaper on sale nearby featured a warning by its editor that the conflict could lead to a "sectarian holocaust".
"To us Egyptians," said writer and analyst Mohammad Al Sayid Said, the sectarian division is "entirely artificial. It resonates with nothing in our culture, nothing in our daily life. It's not part of our social experience, cultural experience or religious experience." But he added: "I think this can devastate the region."
The violence remains confined to Iraq and, on a far smaller scale, Lebanon, but to some, the four-year-long entropy of Iraq offers a metaphor for the forces emerging across the region: People there watched the rise of sectarian identity, railed against it, blamed the United States and others for inflaming it, then were often helpless to stop the descent into bloodshed.
"This tension is the most dangerous problem now in the region," said Ghassan Charbel, editor of the Arabic-language daily Al Hayat.
The schism between Sunnis and Shiites dates to the 7th century, Islam's earliest days, when a dispute broke out over who would succeed the Prophet Mohammad (PBUH).
Shiites believe the descendants of his daughter, Fatima, and son-in-law, Ali, were deprived of divinely ordained leadership in a narrative of martyrdom and injustice that still influences devout Shiite readings of the faith.
Over centuries, differences in ritual, jurisprudence and theology evolved, some of them slight. But the Shiite community — as a majority in Iraq and Bahrain and a sizeable minority in Lebanon, Saudi Arabia and Kuwait — is shaped far more today by the underprivileged status it has often endured in an Arab world that is predominantly Sunni.
Episodes of sectarian conflict litter the region's history: Shiites revolted in medieval Baghdad, and rival gangs ransacked one another's tombs and shrines.
The conflict between the Sunni Ottoman Empire and the Shiite Safavid Empire in Persia was often cast as a sectarian struggle. The 1979 Islamic revolution in Iran was portrayed in parts of the Arab world as a Shiite resurgence.
But rarely has the region witnessed so many events, in so brief a time, that have been so widely interpreted through a sectarian lens: the empowering of Iraq's Shiite-led government and the bloodletting that has devastated the country; the lack of support by America's Sunni Arab allies — Egypt, Jordan and Saudi Arabia — for the Shiite movement Hezbollah in its fight with Israel last summer; and, most decisively, the perception among many Sunni Arabs that Iraq leader Saddam Hussain was lynched by Shiites bent on revenge.
In the background is the growing assertiveness of Shiite Iran as the influence of other traditional regional powers such as Egypt and Saudi Arabia diminishes.
In Lebanon, where the Hezbollah-led opposition has mobilised in an effort to force the government's resignation, the sectarian divide colours even a contest over urban space.
Some Sunnis are angered most by the fact that the Beirut sit-in — in their eyes, an occupation — by Shiites from the hardscrabble southern suburbs is taking place in the sleek downtown rebuilt by a former Sunni prime minister, Rafik Hariri, who was assassinated in 2005.
"Politics is perception," said Jamil Mroue, a Lebanese publisher whose father was Shiite and mother Sunni.
Sentiments today remind him of the tribal-like fanaticism that marked another sectarian conflict, Lebanon's 15-year civil war — which, among other divisions, loosely pitted Christians against Muslims before it ended in 1990.
"It certainly conjures up the feelings of the civil war, when Lebanon started disintegrating, except on a mega-scale," Mroue said. He called it "very scary, because I know that there is a possibility of being moved by this tide."
"At the end of it," he added, "people are going to look back and say, 'What the hell was this all about?'"
In overwhelmingly Sunni countries such as Egypt, where politics were long defined by Arab nationalism or political Islam, visceral notions of sectarian identity remain somewhat alien.
It is not unusual to hear people say they realised only as adults that they were Sunnis. Before that, they identified themselves simply as Muslim.
Even in Lebanon, despite its communal divisions, intermarriage is not uncommon, and there is a long tradition of Sunnis becoming Shiites so their daughters can receive a more equitable share of inheritance, as allowed under Shiite law.
Across the region, Hezbollah and its leader, Hassan Nasrallah, in particular, still win accolades for their performance in last summer's war in Lebanon.
"You have to give him credit for fighting the Israelis," Abdul Hamid Ebrahim said of Nasrallah as he stood at a rickety curbside stand in Cairo, boiling water for tea.
Overhead were pictures of two Egyptian icons, the singers Um Kalthoum and Abdul Halim Hafez. "Closest to my heart," he said. Next to them was a portrait of Nasrallah. "A symbol of resistance, the man who defeated Israel," it read.
"Hassan Nasrallah, he's the man who stood in front of the Israelis himself," said Mohsin Mohammad, a customer.
"Who was standing with him?" Ebrahim asked, nodding his head. He pointed to the sky. "Our Lord."
Both scoffed at the sectarian tensions.
"There's a proverb that says, 'Divide and conquer'," Mohammad said. "Sunnis and Shiites — they're not both Muslims? What divides them? Who wants to divide them? In whose interest is it to divide them?" he asked.
"It's in the West's interest," he answered. "And at the head of it is America and Israel." He paused. "And Britain."
That sense of Western manipulation is often voiced by Shiite clerics and activists, who say the United States incites sectarianism as a way of blunting Iran's influence.
In recent years, some of the most provocative comments have come from America's allies in the region: Egypt's president questioned Shiites' loyalty to their countries, Jordan's king warned of a coming Shiite crescent from Iran to Lebanon, and last month King Abdullah of Saudi Arabia denounced what he called Shiite proselytising.
The charge drew a lengthy retort from Nasrallah. "Frankly speaking, the aim of saying such things is fomenting strife," he said in a speech. He dismissed charges of Iranian proselytising or the emergence of a Shiite crescent.
"There are conflicts in Palestine between Sunni sects — Hamas and Fatah — in Somalia, in Darfur. None of that is sectarian," said Hassan Al Saffar, the most prominent Shiite cleric in Saudi Arabia. "There's a campaign against Shiites. Why is all this anti-Shiite sentiment being inflamed at a time the United States is trying to pressure Iran because of its nuclear ambitions?"
In Cairo recently, Hassan Kamel sipped sweet tea in a cafe beside the shrine to Imam Hussain, the prophet's grandson, who was killed in battle in 680 in what is now Iraq.
The shrine is believed to hold his severed head. Across the street was Al Azhar, one of the foremost academic institutions of Sunni Islam, founded, ironically, by the Shiite Fatimid dynasty that ruled Egypt for 200 years until 1169.
On the shrine's wall was a saying attributed to the Prophet and often intoned during Shiite commemorations: "Hussain is from me, and I am from Hussain."
Kamel pointed to the doors, topped with a Quranic inscription; Shiites and Sunnis like him worshipped at the shrine together, he said.
He wondered aloud about past conflicts that have splintered the Middle East.
"Egyptians, all their lives, without exception, have endured so many crises, catastrophes and problems," he said. He listed wars in 1948, 1956, 1967, 1973. "But they have a gift. It's a gift from God. They have the ability to forget."
Then he talked about the rest of the region, and whether this bout of strife and tension would pass, too. "They might forget, they might not," he said. "Right now, no one knows what's coming."