The police in Northern Ireland have described a bomb left near Londonderry courthouse as a "substantial viable device".
District Commander, Stephen Martin, said a beer keg, left in a stolen car, contained around 50kg of home-made explosives.
He said he believed it was left by dissident republicans, and could have caused death or serious injury.
Dozens of residents were moved from sheltered housing during the alert.
Fifteen choirboys were told to get out of the nearby St Columb's Cathedral.
The alert followed a telephone bomb warning which was received at about 1845 BST on Sunday.
District Commander Martin said he expected a dissident republican group to claim it had carried out the attack over the next number of days.
"In my view it will be either the Real IRA or Oglaigh na hEireann," he said.
The police, he said, would try to return the area to normal by the mid afternoon, but they would be "cautious".
"If a bomb such as this had gone off it would have killed or seriously injured people," he said.
"It would have caused substantial damage to nearby property; to the courthouse, to the Masonic Hall, to the Deanery, to other residents.
"It would have probably damaged the residential home, Alexander House.
"It's just a highly built up residential part of the city within the walls and a device this size would have caused considerable devastation."
He also said that police patrols had been increased in response to a perceived increase in the risk of violence.
"It's two years since the threat in Northern Ireland has been escalated to severe and acts like this are a strong possibility," he said.
"They can come at any time without warning."
Dean William Morton of St Columb's Cathedral described the incident as a "logistical nightmare".
He contacted parents of choirboys, some of them as young as seven, who were at the Cathedral for a rehearsal for the opera, Tosca.
"It's achieving absolutely nothing other than galvanising the resolve on the part of the huge majority of people here to move ahead," he said.
"I am sure it will all be cleared and life will continue."
Community worker, Charlie McMenamin, helped to transport the residents of Alexander House to another sheltered housing facility.
"Obviously being elderly they're upset," he said.
"They're not used to this situation where they're being moved around without any sort of notice.
He said one woman had told him, "I am sorry I moved to Derry."
The residents have now returned to their home.
Sinn Fein MLA, Martina Anderson, said the device was out of step with the views of the vast majority of the people of the city.
"We are all of the view that an attack on any place in this city is an attack on us all."
Angry and frustrated
SDLP representative Pat Ramsey said the residents who had to leave Alexander House, were "distressed, angry and frustrated" at what had happened.
"Even with people who have not been evacuated there is a sense of confusion, because in the city centre there are a lot of residential areas," he added.
The DUP's William Hay appealed for whatever resources were necessary in dealing with the threat of violence to be made available to the PSNI.
"There's no doubt about it," he said, "where it was placed it was to cause maximum damage to property and lives."
The UUP Councillor, Mary Hamilton, condemned the bomb.
"We are trying to move on and look towards a city of culture and this is certainly not helping."
The security alert is ongoing, with traffic being diverted away from the Bishop Street area.
Court hearings at the courthouse on Monday have been moved to other courts.