Exactly one year since a bomb exploded in the busy town centre, claiming the lives of 29 people, a special prayer is to be read at all morning church services.

An inter-denominational memorial service is to be held in front of the town's courthouse at 1445 BST, with a minute's silence at the exact time of the explosion.

The service takes place against a background of tension elsewhere in Northern Ireland, and strains in the peace process.

Clashes follow parade

There were clashes late on Saturday night, and early on Sunday morning in the centre of Londonderry following a march by the Protestant Apprentice Boys.

The BBC's Denis Murray reports: "The bombing has brought people togther"

At one stage a crowd of between two and three hundred people gathered in the city centre, and the police say a number of petrol bombs were thrown.

Several buildings, including three banks, were badly damaged by fire.

The police say most of the shops in the area have also been looted. Several vehicles were set alight, while firefighters came under attack and had difficulty getting into the area.

There were ugly confrontations as police in riot gear moved in to clear the city centre.

At Sunday's service in Omagh, prayers will be read by visitors from Buncrana, Co Donegal, and Spain, where some of the victims came from.

The service has taken a while to plan out of concern for the bereaved and survivors, says the BBC's Ireland Correspondent Denis Murray.

Some families have still not decided whether or not to attend.

Floral tributes

The BBC's Peter Hunt: "This is a difficult time for the people of Omagh"

Townspeople also held a minute's silence on Saturday, the eve of the anniversary.

Omagh came to a standstill shortly for the brief ceremony at 1510 BST, the precise time of the explosion.

Hundreds of people stood in silence and shops were closed as a mark of respect.

The bomb left a scene of devastation

Among the hundreds of floral tributes and messages of remembrance already placed in the garden on Market Street were the anonymous thoughts of one Omagh resident.

"To all those injured and to the 29 innocent people who lost their lives in a horrific bomb on a busy Saturday afternoon on August 15 1998.

"All this carnage and oppressive feelings were all caused deliberately by other members of our population whose feelings of bitterness, covetousness, envy, ill will, spite, rivalry and jealousy overcame them.

"Whoever said that there was going to be peace?"

The republican splinter group, the Real IRA admitted planting the bomb.

Messages of sympathy

The BBC's Denis Murray: "Service a long time in the planning"

In a message of sympathy to the bereaved, Irish Prime Minister Bertie Ahern said the British and Irish governments would work together "to bring about a situation where no other family in these islands or further afield experiences the grief and loss which you have suffered".

The British Prime Minister Tony Blair said:"The whole community in Northern Ireland and indeed, across the world, has felt moved to reach out to those affected by the bombing in Omagh.

"The pain for many is as yet undiminished, but I assure them their suffering will not be forgotten."

Irish Foreign Minister David Andrews pledged that on both sides of the border, police would continue to work towards rounding up those behind the attack.

In a statement, he said: "We should all re-dedicate ourselves to the full implementation of the Good Friday Agreement on Northern Ireland and must in the forthcoming weeks renew our efforts to overcome the remaining obstacles to its success."

He said the appalling death and suffering caused by the Omagh bombing had "evoked horror, outrage and deep sympathy throughout these islands and across the world".

"We must never forget the 29 who died, nor the pain of the injured or the grief of the bereaved, whether in and around Omagh itself, in Buncrana or in Spain."

Two Spanish exchange students were among those killed in the blast.