CAIRO (AP) — Early in the morning two Sundays ago, hundreds of Christian Egyptians quietly slipped into a former underwear factory where they had discreetly set up a church and held their first service. Bells rang and hymns were sung.

A crowd of angry Muslims quickly gathered, threw stones at the building and burned banners that said, "No to the church."

They tried to storm the gates, clashed with police and chanted, "The church has fallen, the priest is dead," according to witnesses.

In fact, no one died, but 13 people were reported injured. For Egyptians in general, the incident in the blue-collar district of Ain Shams served as a warning that Muslim-Christian clashes, largely confined to the south of the country in recent years, have seeped into the capital.

Tempers are flaring as Islamic conservatism gains ground and Christians grow increasingly resentful about discrimination by the Muslim majority.

The Ain Shams incident highlights that even in Cairo — seen as more cosmopolitan in its sectarian relations than the rural south — suspicions run between the communities.

Muslim and Christian neighbours also are competing over who can set up houses of worship and where. "We don't want to hear their hymns and for sure, they don't want to hear our prayers," a Muslim woman who lives in the area said as she shopped at a dairy store.

Like other residents, Muslim and Christian, she spoke to The Associated Press on condition of anonymity, fearing trouble with police standing nearby.

Security agencies often try to prevent discussion of sectarian tensions.

Egypt's government often tries to play down such strains. Police responding to incidents in southern villages usually only intervene when violence gets out of control, often arresting an equal number of people from each religious group.

Then authorities force the families involved to reach a behind-the-scenes compromise, without investigating or prosecuting aggressors.

Critics say that only papers over the causes of tensions and say the government is bending to fears among Muslim conservatives that Christians are seeking to undermine what they see as Egypt's Muslim identity.

Those fears lie at the heart of disputes over the building of new churches. Two incidents this summer underscore the problem.

In one southern city, a Muslim man was killed in clashes over the expansion of a Coptic Orthodox monastery, and Muslims torched Christian villagers' homes because a priest was seen holding Mass inside a house, according to the Egyptian Initiative for Personal Rights, a rights group.

Christians, an estimated ten percent of Egypt's 79 million people, have long complained of government restrictions on building new churches.

To build a church or even renovate an existing one, clearance is needed from several security agencies and government bodies, and often is refused.

A church cannot be built near a mosque, but "near" is not defined.

And nothing prevents Muslims from building a mosque near a church, even without a permit.

As a result, most of Cairo's churches are surrounded by mosques, often bigger and taller.

Egyptian Christians don't have enough churches to accommodate their numbers, so they hold informal services in community centres, bookstores or homes.

"There is this psychological terrorism from Islamists that prevents local authorities from demolishing illegally built mosques and complicates permit procedures for Copts," says Youssef Sidhom, the editor of Watani, a newspaper run by members of the Coptic Orthodox Church, Egypt's main Christian denomination.

In Ain Shams, where about 4,000 Christian families are vastly outnumbered by Muslims, congregants bought the factory three years ago and quietly began setting up their church.

They say they needed a new facility because their 40-year-old Church of the Virgin Mary nearby could only accommodate a quarter of the congregation.

According to Father Anthony, the priest who led the church's first and only service so far, everything was done quietly. No crosses or other religious symbols were put on the exterior.

A Muslim resident who gave her name as Umm Toqa said she never realised what was going on in the building. "I thought that there were ghosts inside as lights turned on at night. We could only hear sounds but saw nobody," she said.