This ancient, walled city is poised on the brink of sectarian violence, despite an attempt by the British government to keep Protestant unionists and Catholic nationalists apart for the climax of the Protestant marching season.
Using barbed wire, corrugated iron panels, and military vehicles bristling with high-tech weaponry, British troops and police have sealed off a section of Londonderry's old, encircling walls, which overlooks the fiercely nationalist Catholic Bogside area.
The barricades sever the planned route of Protestant marchers from the Apprentice Boys of Derry, who had sought to parade the entire length of Londonderry's historic walls on Saturday.
All of Northern Ireland is now looking fearfully to Londonderry, known as Derry to Irish nationalists, which has for years been an informal barometer of sectarian feeling.
Tens of thousands of nationalists and unionists are expected to pour into the city for the Protestant parade on Saturday, and a planned Catholic protest march the night before.
The city has been humming with dark rumors of violent confrontation after talks failed between the Apprentice Boys and the Bogside Residents' Group, which claims to represent local Catholics.
"It's really so scary," says Liz McConnell, a young Catholic mother of four who attended the failed meetings. "It's like a bomb ticking away, every day that the parade comes closer."
Glancing out her living room window, in a small Bogside house tucked beneath the city walls, she says: "This year we're getting our daughters out before any trouble starts. They'll stay with their granny about a mile away until things calm down."
Britain's Northern Ireland Secretary, Sir Patrick Mayhew, says he decided to close the disputed section of wall to help the Royal Ulster Constabulary (RUC) control an increasingly volatile standoff. "When you have conflicting rights, it's the function of the police to try to maintain order, and it's the function of everyone of goodwill to see that calm and good order are maintained in a very difficult and tense situation," he says, insisting that the decision was not designed to favor or insult either community.
But in true Ulster style, both sides are giving vent to righteous outrage. Catholics in the Bogside accuse the British government of "militarizing" an already dangerous situation. Unionists construe the wall closure as a victory for Irish nationalists.
"How are Protestant people expected to react?" asks an outraged Democratic Unionist councilor, Gregory Campbell. "In the past, street protests have won nationalists their own way," he says, "Don't be surprised if Protestants learn from that."
Many Catholics argue that Protestants learned the efficacy of grass-roots protest last month, when, after an five-day standoff, hard-line unionists faced down the RUC in the village of Drumcree, and marched down a Catholic road.
The crisis in Drumcree sparked some of the most serious rioting ever seen in Londonderry. One man was killed and thousands of dollars of damage was done. People say it poisoned attitudes on both sides of the sectarian divide.
"They think they have an absolute right to march where and when they want, says Donnacha MacNaillais, the controversial leader of the Bogside Residents' Group who has served time in prison for weapons possession.
"If they feel this weekend's march is going to lead to confrontation," Mr. MacNaillais suggests coolly, "they should cancel it."
Beneath the fluttering red, white, and blue pennants strung across the streets of the Fountain, a small Protestant enclave near the Bogside, there is a palpable sense of defiance under siege.
"We are not trying to be offensive to anyone," says Alistair Simpson, governor of the Apprentice Boys of Derry, a society formed in the early 19th century. "All we ask is the civil right to march and celebrate our culture and our heritage."
Each year members march to commemorate the heroic closing of the city gates in 1689 by 13 apprentice boys, as the Catholic forces of King James II bore down on the city.
Outside Mr. Simpson's office, young men are dragging old mattresses, tree branches, and broken wooden pallets to create what will become an enormous bonfire at midnight tonight.
Police and residents fear the first flashpoint may come late tonight, when five Catholic protest parades are due to converge on the city center at the same time the Protestant celebrations begin.
Margaret Logan, a Protestant teacher on her way through the Fountain, believes "certain elements" in the Bogside - Sinn Fein and the Irish Republican Army - are hoping the protest parades will provoke another Drumcree-style standoff.
"This has all been organized, it just didn't come out of the blue," she says, echoing an often-heard opinion here. "The [Catholics] want to see confrontation."
The Apprentice Boys parade along the city walls in 1969 sparked rioting in the Bogside, triggering the first arrival of British troops in the province.