Hamilton described how a pizza delivery driver was hijacked on Saturday evening and his auto was packed with explosives before being left outside the court house on Bishop Street in Derry.
Assistant Chief Constable Mark Hamilton said the device was "to some degree effective", adding that it "wasn't so effective that it created widespread damage".
"Bishop Street closed. Stay away".
"In the intervening minutes we had already found the auto and started to evacuate the area".
Police received a telephone tip-off that the device had been planted and an evacuation was nearing completion when the explosion occurred.
Police officers guard the scene of a suspected vehicle bomb in Londonderry, Northern Ireland, on January 20, 2019. Picture Margaret McLaughlin 19-1-2019.
A number of church services in the area have been cancelled due to the ongoing security operation. If you live Bishop Street or Bishop Street Without look at potentially making preparations to leave.
Despite the explosion, she went ahead to the party. She told the Irish News it was "all a bit scary".
In a post on social media, PSNI Foyle said: "Folks we are responding to a report of an abandoned vehicle on the Northland Road close to the Glenbank Road junction. We were just told to stay where we were; the police were trying to find out what was going on".More news: Trump to make announcement on shutdown
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The incident has been condemned by political parties across the traditional republican and unionist divide in the province, as well as by leaders in the Republic of Ireland.
U.K. Northern Ireland Secretary Karen Bradley said the dissidents' "campaign of hatred and violence is unfortunately nothing new", but wouldn't succeed.
Uncertainty about the future of the Irish border after Brexit is adding to tensions in Northern Ireland.
"I know this second security alert and the measures we are taking to keep people safe will cause significant inconvenience for the local community, but we have no choice".
Derry Sinn Féin MP, Elisha McCallion called on anyone with information about the attack to pass it on to police.
"Derry is a city moving forward and no one wants this type of incident".
PSNI officers cut gates at the back of a youth club to allow 150 people who were there to take part in a charity quiz to enable them to escape from the building onto the city's walls.
Derry's SDLP mayor John Boyle challenged those responsible on what the aim was. I utterly condemn this attack which could easily have resulted in loss of life or injury.