A suspected car bomb has exploded near Bishop Street courthouse in Londonderry, sparking a major police operation.
The area has been sealed off following the explosion on Saturday evening and a number of homes and bars nearby have been evacuated.
It is understood no one was injured in the blast, but police have said they are concerned about a second vehicle in the area.
People have been warned to stay away from the scene.
"There is another car we are not happy about," PSNI Foyle posted on Facebook.
"There are ongoing necessary evacuations.
"If you live Bishop Street or Bishop Street Without, look at potentially making preparations to leave. We can only apologise for any inconvenience."
Sinn Féin MP Elisha McCallion has condemned the apparent attack, saying elderly residents had been particularly alarmed.
"Derry is a city moving forward and no one wants this type of incident. It is not representative of the city," she added.
"I would encourage anyone with information about this incident to bring it to the police."
SDLP Foyle MLA Mark H Durkan tweeted: "Whoever is responsible for this explosion outside Bishop Street Courthouse in Derry clearly hasn't got the message that the people of Derry DO NOT want this on our streets.
"We are trying to move Derry forward and will not let anyone drag us back to the dark days of the past."
DUP leader Arlene Foster described the incident as a "pointless act of terror" which must be "condemned in the strongest terms".
She added that it had been "perpetrated by people with no regard for life".
Derry City Mayor John Boyle said the perpetrators did not speak for local people, adding that he was "absolutely appalled by this terrible act of violence tonight right in the heart of our city".
He further said: "I utterly condemn this attack which could easily have resulted in loss of life or injury."
Tánaiste Simon Coveney has also condemned the incident.
"There is no place and no justification possible for such acts of terror, which seek to drag Northern Ireland back to violence and conflict," he said.