Police in Londonderry have arrested two men in their 20s in connection with a bomb in the city on Saturday.
They said the attack may have been carried out by dissident republican group the New IRA.
A pizza delivery van was hijacked by two armed men in Derry at about 18:00 GMT.
The bomb, which went off at 20:09 GMT, was described as a “crude device”. The PSNI said the attack outside the courthouse was “unbelievably reckless”.
Assistant Chief Constable Mark Hamilton said the van, with the bomb inside, was left outside the courthouse on Bishop Street at 19:23 GMT.
Three minutes later, a warning was called into the Samaritans in the West Midlands. It was passed to West Midlands police, who contacted the Police Service of Northern Ireland.
“In the intervening minutes we had already found the car and were starting to evacuate the area,” said ACC Hamilton.
“Clearly, it was a very significant attempt to kill people here in this community.
“Thankfully, the local community and the police service acted bravely together and we got everybody away just in time.
“But the bomb detonated just as we were leaving the area.”
A cordon remains in place at the scene.
Residents, hundreds of hotel guests, 150 people from the Masonic Hall and a large number of children from a church youth club were evacuated.
Greg McLaughlin, who lives nearby, said his windows shook with the force of the blast.
“It was very, very loud. I knew right away this was a bomb,” he said.
“We knew it was quite close.
“You could see the ball of fire on the street. It sounded to me like a very significant blast. I haven’t heard anything like it in Derry for quite a while.”
The Northern Ireland secretary, Karen Bradley, said the bomb was an “attempt to disrupt progress in Northern Ireland”.
“The small number of people responsible have absolutely nothing to offer Northern Ireland’s future and will not prevail.”
The Church of Ireland Bishop of Derry said the bomb was “was an act of utter recklessness”.
“The attack was wrong. It is indefensible. It should never be repeated. The time has long since passed when such violence should have been consigned to history,” the Right Reverend Ken Good said.
Colin Neill, chief executive of Hospitality Ulster, said businesses were determined to trade as normal despite disruption in the area.
“We are a resilient bunch in the hospitality sector and this incident last night will not deter us from opening today and getting on with the job.”
He said businesses had bounced back and would continue to welcome “visitors and tourists back to the city”.
Sinn Féin MP Elisha McCallion said the incident had “shocked the local community”.
“In particular, there are many elderly residents who live in the area who have been alarmed by this incident,” she said.
SDLP leader Colum Eastwood said those responsible “represent no-one”.
“They profess to be fighting for Ireland against the British state,” he said.
“What they fail to understand is that they stand in opposition to the Irish people. That is a battle that they can never win.”
The DUP MLA, Gary Middleton, described the incident as a “disgraceful act of terrorism”.
The Irish tánaiste (deputy prime minister) Simon Coveney condemned the attack.
“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 wrote on Twitter.