Sometimes before a horrific event there is a warning. Nothing huge, just a small signal that things are not quite right. A strange atmosphere. A sense of foreboding. A feeling of impending doom.

But nobody felt that in Omagh last August 15th.

The town centre was always bustling on a Saturday, but this one is particularly busy. A festival is taking place with floats and music. Everybody is in a relaxed, happy mood. Dozens of tourists, including a Spanish party, arrive to watch the celebrations.

Market Street is bustling with shoppers, particularly women and children. Many are in town to buy school uniforms for the start of term. In the cafes, teenage girls gossip with their friends about boys and where they will go and what they will wear that night. Many of the men have left the women shopping and gone to the pub. It is the start of the Premier League and they are keen to see the football.

At 2.30 p.m., telephone calls to a Belfast newsroom and then to the Samaritans warn of a bomb in Omagh.

There is still confusion today about the exact details of the warnings. But RUC officers on the ground tell shoppers they believe there is a bomb at the courthouse at the top end of High Street.

They start evacuating that area and move the people to the bottom end of High Street, which runs into Market Street.

Shoppers complain about the inconvenience and do not take the warnings very seriously. Staff in at least one shop refuse to leave the premises.

"Nobody was panicking," says Claire Gallagher. "Nothing ever happened in Omagh."

Thirty minutes after the first bomb warning, hundreds of people have congregated at the bottom of Market Street and the Dublin Road. The mood is still relaxed.

At 3.10 p.m. a bomb explodes in a maroon Vauxhall Astra at the junction of Market Street and the Dublin Road. The surrounding buildings collapse. Nichola Donnelly does not even hear the blast.

"I was outside McElroy's shoe shop when the bomb went off. I was blown through the window and into the shop. I didn't hear the explosion. I just felt the force of it and the rubble landing all around me. I saw nothing but red, yellow and orange lights, like sparklers. I felt a very strong heat. And there was a powerful smell, the smell of burning."

For a few seconds after the bomb there is silence. Then it begins, the wailing and screaming. Those who can still walk, like Nichola, push their way out of the rubble. Others remain buried underneath.

Families and friends together only a few moments earlier have been split apart. People search frantically through the debris for their loved ones. Names are shouted in desperation. Dozens of car alarms have been set off. The devastation is horrific.

Hands, arms and legs litter the road. Many people are hysterical. Children cry uncontrollably for their parents.

There are bodies everywhere. It is often difficult to distinguish the dead from the living. The street is strewn with glass, rubble, and human limbs. The pavement is spattered with blood. Some people who escaped injury are not in a fit state to help. They just wander, dazed, through the streets.

Others join the rescue operation. Bodies are carried on doors, boards, benches and other makeshift stretchers. The injured are taken to hospital. The dead are laid on the side of the road.

They are covered with tarpaulin or plastic sheeting. When that runs out, quilts, curtains and sheets from the nearby drapers are used. People are saying the Last Rites over the dead.

A water main bursts and water flows over the bodies. One man who arrived minutes after the explosion says he never witnessed scenes like it in his life. "I heard the bang and rushed in but at first I didn't know what had happened. I thought it was dummies from the drapery shop lying in the middle of the road and then I realised it was real bodies."

Hundreds of people who heard the blast from a few miles away have arrived to search for friends and relatives they fear could have been caught in it. Towels, coats, cardigans and handkerchiefs are used to bandage the wounded.

Ambulance crews and paramedics are on the scene. Bodies are placed in bags, zipped up, and taken away. Staff who were off-duty flock to Tyrone County Hospital. Four extra wards are opened but it is still not enough and British army helicopters have to fly some of the injured to hospitals in Belfast, Derry, Enniskillen and Dungannon.

Electric cutting equipment has been brought in to help in the search. The whining noise from the machinery mingles eerily with the sound of ambulance sirens.

Radio and television bulletins have broken news of the blast. People from all over Ireland and the rest of the world try to contact Omagh. The telephone system cannot cope and lines are jammed.

The scenes at Tyrone County Hospital are even worse than in the town centre. A trail of blood leads up the steps. There is blood on the walls, on the floors and in the toilets. A priest gives the last rites to people lying on mattresses in the corridor.

Staff members fight back tears. "I want to walk away. I don't want to deal with this but I know I have to," says a nurse.

There are heart-breaking scenes in the hospital when some families learn that their relatives are dead. For others there is joy when they are told their loved one is only wounded. They do not even inquire about the injuries. That will come later. It is being alive that matters when all around there is death and destruction.

Bit by bit the individual stories seep out. Best friends died together. Lorraine Wilson (15) and Samantha McFarland (17), volunteers in the local Oxfam shop, were evacuated from the building and walked straight into the blast.

Family members died side by side. Mary Grimes (65) went into town on her birthday with her daughter, Avril Monaghan (30), who was pregnant with twins, and her baby granddaughter, Maura. They were buying cloth in Kells's drapery shop when the bomb went off. Mary was so badly mutilated she could be identified only by her fingerprints.

Those killed came from all classes, creeds and age groups. The youngest was 18 months and the oldest 65 years.

There was Esther Gibson (36), a Sunday school teacher for the Rev Ian Paisley's Free Presbyterian Church, and Brenda Logue (17), who lived for her local Gaelic football team. Some of the victims were friends and neighbours.

The one thing that united them was that they were all in Omagh at 3.10 p.m. a year ago today, on an ordinary Saturday afternoon.