Sister Sugiyanto's guided teatime tour of Temple Square, the world headquarters of the Mormon Church, was rudely interrupted by the sound of emergency sirens and police helicopters hovering over central Salt Lake City.

A suspicious package containing white powder had been opened by a clerk in the Church Administration Office, prompting FBI agents wearing chemical warfare suits to swiftly evacuate the building. Across town, news was coming in that eight local churches had been vandalised. One, in a family neighbourhood, had obscene graffiti scrawled on its walls. The other seven, in the nearby towns of Layton and Ogden, had windows shot out, apparently with a BB gun.

The brouhaha on Thursday was severely testing the happy demeanour of the sister, a visiting missionary from Indonesia whose informative trips round the Mormon Church's 45-acre HQ culminate in a not-so-subtle attempt to recruit you. "I feel we are being picked on," said Sister Sugiyanto. "We are not the only group that supported this proposition, so why do they only blame us? Last week, thousands came here to protest. It made me sad, more than anything."

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The proposition in question is Proposition 8, a ballot measure outlawing same-sex marriage in California that was backed by 52 per cent of voters on 4 November. The "they" refers to gay rights activists upset by the Mormon Church's role in the campaign to push the measure through. Modern morality and religious doctrine have collided in spectacular fashion, and nowhere more so than here in Utah.

To liberal America, the Church of the Latter Day Saints and its 12 million members around the world are suddenly public enemy number one. They stand accused – and, it must be said, it is an accusation they strongly deny – of thinly-veiled homophobia, using their massive financial muscle to help railroad the ballot measure.

In California, where 18,000 recently-married gays and lesbians, including the chatshow host Ellen DeGeneres and the Star Trek actor George Takkei, have been thrown into legal limbo, anger has reached fever pitch. Dozens of Hollywood stars have backed a campaign to name-and-shame Mormon-owned businesses that financed Proposition 8. Others have attended protests at temples, severely testing the diplomatic skills of Mormon performers such as Donny Osmond and Brandon Flowers, the lead singer of The Killers. More gay rights demonstrations will be held at church properties across the US today.

Utah, where more than a million Mormons reside (they are the majority religion in the state) is now facing a consumer boycott that threatens to disrupt its $6bn (£4bn) winter ski season and may even affect January's Sundance film festival.

Salt Lake City sits in the middle of this storm. It was built by Brigham Young, a Mormon leader who fled west in the 1840s after suffering religious persecution and today, the city's inhabitants greet their new troubles with steely resolve.

"This is not democracy. This is not American. This is terrorism, for want of a better word," said Renee Scheffers, a guest at a wedding at the Salt Lake Temple, the imposing granite building that, together with the snow-capped Wasatch mountains, dominates the city's skyline. "The gays have become everything they accused their opponents of," added her fellow guest, Wilson Clyde. "They're intolerant of me, of my beliefs and my way of life. They're nothing more than extremists who are trying to intimidate and silence anyone who disagrees with them."

The debate has certainly turned ugly. In addition to the "anthrax" scares that saw Temple Square, together with a Mormon temple in Los Angeles, evacuated on Thursday (tests later showed that the white powder was not toxic), attacks on other Mormon properties have raised a chilling spectre of religious hatred. In one a charred copy of the Book of Mormon, which members study in addition to the Bible, was found on the steps of a church in Colorado.

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For Mormon leaders, it's another chapter in a long history of vilification that sees them wrongly portrayed as teetotal, right-wing oddballs, or freakish polygamists.

"It's easier to attack a minority religion, especially one that frankly isn't very well understood, than to protest because 70 per cent of African American voters also supported Proposition 8," says Mike Otterson, a somewhat exasperated church spokesman. "It's a tactical thing. It makes it easier for them to vent their anger and frustration. But to vandalise chapels, vandalise temples, put graffiti on our buildings, protest outside our temples ... It's completely unreasonable. People have the right to protest. But this is way over the top."

The facts regarding the current round of protests are disputed by both sides. The disagreement started several months before polling day, when Mormon congregations were read a letter by Thomas Monson, the Church's president or "prophet" (a sort of Mormon Pope) asking them to give time and money to help ban gay marriage.

Followers, who place the family at the heart of their faith, responded in their thousands, providing up to $40m by some estimates that allowed the "Yes on 8" campaign to run a series of aggressive and highly-successful television attack ads. Some featured small girls announcing: "Today I learned in school that a prince can marry a prince and I can one day marry a princess!"

That campaign aggravated those who already harboured misgivings about the Church's financial clout, puritanical streak, and influence on public affairs. Many influential politicians are Mormons, including the former Republican presidential contender Mitt Romney, and the prominent Democrat Harry Reid.

In Utah, to the dismay of non-religious residents, the Church's influence on public affairs means the state boasts the most stringent alcohol licensing laws in the US, and is the only state in the US where it remains illegal to gamble.

Yet, despite the controversy, the real face of Mormonism is reassuringly cuddly. Church members may not necessarily be perfect company at a dinner party (they are required to forswear alcohol, tobacco, tea and coffee) but they tend to be sympathetic souls who would make agreeable next-door neighbours.

Even the local gay pride office has some words of praise for the Mormon Church: "In a lot of communities prejudice airs itself in a violent way, in an outward way," says a spokeswoman, Maria Gomberg. "Here in Utah, although we do experience hate crimes, the LDS Church generally fights through legal action."

To the fascination of the rest of the world, a few members still practise polygamy (an estimated 30,000 in Utah), but it is officially frowned upon by the Church, who renounced it as a condition of Utah gaining statehood in the late 19th century. The public face of the practice is now limited to television shows such as Big Love, and fundamentalist breakaway factions, including the Texan sect that was raided by child-support services this year.

Mormons believe in a version of Christianity that stems from the Book of Mormon, which the study in addition to the Bible. This text was allegedly recorded on gold tablets by inhabitants of North America shortly after the time of Christ. The tablets were discovered in the 1820s – or so the story goes – by the religion's founder, Joseph Smith.

Smith's own standing as a polygamist has laced the argument over gay versus "traditional" marriage with a heavy dose of irony. Among members of Salt Lake City's surprisingly active gay community, it is a source of mild amusement. "We have actually opened a gay bar in the middle of a polygamist compound," says Brian Morriss, owner of the newly-built hostelry Jam in the city's Marmalade district. "Just after we leased the land, we discovered that a family called the Kingstons owned three buildings around us. The father has 106 children."

Though Mr Morriss and Todd Croft, his business (and romantic) partner, joined their patrons at noisy anti-Mormon protests at Temple Square, they say Salt Lake City is a surprisingly hospitable place for the gay community. "It is the third largest gay city per capita in the west, after Los Angeles and San Francisco. Thirty-thousand come to our gay pride events," he says. "I have a lot of Mormon friends, and they are not as extreme as everyone thinks. In fact, they are mostly apologetic about what their church has done."

Scott McCoy, an openly-gay state senator whose election lays bare the metropolitan nature of at least some areas of Salt Lake City, says that any boycott of his state will merely cause the Church to shift further to the right. "By saying we're going to build a big wall around Utah, well for folks who live here who are trying to make changes, it just leaves us high and dry. Mormons further to the right would be delighted if gay people boycotted Utah, because they don't want gay people here."

The future of the debate may therefore lie in the hands of moderate Mormons who recognise the potential PR problems recent weeks have brought to their church and who say the time is right for a rapprochement. "I speak only for myself, and not the Church, but I am in favour of domestic partnerships," says Paul Pugmire, a PR consultant and prominent figure in local Mormon circles. "This would include gay couples. It would include a pair of unmarried sisters living in my neighbourhood, it would include any number of different non-traditional families.

"Marriage is a bright line I don't think we'll ever cross. But there is a great deal leading up to that bright line which really should be examined. We now have a great opportunity to come together on this. There is so much more that can be done, and if recent events have taught us anything it is that we must end the hate, for everyone's sake."

Fact file: The Mormons



*The Church of Jesus Christ of Latter-Day Saints was founded by Joseph Smith, a farmer's son from Vermont. Smith began to gather a religious following after announcing that an angel had shown him a set of golden plates describing a visit of Jesus to the indigenous peoples of the Americas. In 1830, he published what he said was a translation of these plates as the Book of Mormon. The same year, he organised a new church – the Church of Christ.

*Smith came up with 13 core beliefs or "Articles of Faith" for his church. One says: "We believe in being honest, true, chaste, benevolent, virtuous and in doing good to all men..."



*The Mormon church, based in Salt Lake City, Utah, has 12 million members worldwide – 190,000 in Britain, where it has operated since 1837.



*Jesus Christ is at the centre of Mormon belief, and members say He will one day return to earth to reign. They think conventional Christian churches have lost God's authority.



*Mormonism focuses on traditional family values. Members oppose abortion, homosexuality, sex outside marriage, pornography, gambling, tobacco, tea, alcohol, coffee and drug use.