The death of a 25-year-old police officer in a car bomb blast has brought tragedy to Omagh, Northern Ireland Chief Constable Matt Baggott has said.
He paid tribute to "modern day hero" Constable Ronan Kerr, a new Catholic recruit, killed outside his home.
No group has said that it carried out Saturday's bombing, but suspicion has fallen on dissident Republicans.
Republican and Unionist politicians have condemned the attack, which David Cameron called "wicked and cowardly".
Constable Kerr is the second policeman to be killed since the Police Service of Northern Ireland was formed out of the Royal Ulster Constabulary in 2001.
He had joined the police in May 2010, and had been working in the community since December. Prayers were said for him at vigil masses throughout Omagh on Saturday night.
Pat Noma, who is a friend of Constable Kerr's mother, said she had also lost her husband to illness relatively recently.
"Ronan was her rock and her strength and he was the one she depended on to get her through it and this young man - who was an only a child - and now this has happened and it's just so devastating," she said.
"All I can say is that I'm calling on all the mothers in Omagh to come out on her behalf. We've got to find these people we've got to stop them from doing this.
"Nobody in Omagh wants this to happen and we've got to call everybody together to hand them over. We can't tolerate this. Omagh has suffered enough."
Chief Constable Baggott said: "Tragedy has returned to Omagh with the loss of constable Ronan Kerr. We've lost one of our brave and courageous peacekeepers.
"All of our thoughts and prayers tonight are with the Kerr family. He was, in my eyes, a modern day hero."
District Commander Temporary Superintendent, Pauline Shields, said: "My colleagues are deeply upset, deeply hurt, by what has happened this evening and our sympathies go to Ronan's family.
"But I can assure his family and his colleagues and the wider community that we will do everything in our power to not only bring to justice those who committed this act but to serve all the community impartially and professionally, as Ronan had done in the exemplary service he had provided in this district."
About 2,000 people, who were taking part in the Omagh half marathon, had passed the nearby entrance to the estate just hours before the blast in Highfield Close.
Northern Ireland First Minister and DUP leader, Peter Robinson, said he shared the outrage of the country.
"It was a young man who was bravely entering the police service, recognising that he was putting his life on the line.
"I have absolutely no doubt the overwhelming number of people in Northern Ireland want to move on. It's only a few Neanderthals who want to go back. They will not drag us back to the past."
'Dark and bloody past'
Deputy First Minister Sinn Fein's Martin McGuinness said those who had carried out the attack had "betrayed the community".
"In joining the police service this young man sought to protect and serve the community and to be part of defining a better future for all of us," he said.
Prime Minister David Cameron said: "Those who carried out this wicked and cowardly crime will never succeed in dragging Northern Ireland back to a dark and bloody past."
SDLP Chairman Joe Byrne, who was at the scene in Omagh, said it was a brutal attack and those responsible had no support in the town.
"Omagh is a mixed town and we're proud of our Catholic police officers. The amount of anger and stunned sadness from across every section of the community shows that," he said.
Since 2007, dissident republicans have planted several booby-trap bombs under the private cars of police officers.
Most bombs have failed to detonate, but in January 2010 Constable Peadar Heffron, 33, was injured when a bomb exploded under his car. He later lost his leg. Dissident republicans had been blamed for the attack
In March 2009, a police officer was shot dead as he answered a distress call in Craigavon, County Armagh.
Constable Stephen Carroll, 48, from Banbridge, was murdered two days after the Real IRA shot dead two soldiers outside Massereene Barracks in Antrim.
His widow, Kate Carroll, called the attack on Constable Kerr a "dastardly, cowardly act", and called on the perpetrators of this latest killing to own up.
"They wanted Catholics to join police. Now that the Catholics are joining up, they are shooting, killing them. What else do they want? Blood?
"I know exactly what the family are going to go through - absolute devastation."
The Continuity IRA claimed it had killed Constable Carroll. It is one of a number of dissident republican paramilitary groups opposed to the peace process. They have carried out bomb and gun attacks on civilians and the security forces.
There is believed to be cross-over and co-operation between the Continuity IRA and the larger Real IRA, which bombed Omagh in 1998.
The car bomb killed 29 people, including a woman pregnant with twins, and injured hundreds more.
Michael Gallagher's 21-year-old son, Aiden, was killed when the Real IRA bombed Omagh's town centre. He said the people of Omagh are being denied justice.
"These people came here 12 and a half years ago and they murdered 31 people, including two unborn children, with impunity. Not one person had been brought to justice for that.
"So why shouldn't they think that they could come in this afternoon and murder someone else?"