Latest: Detectives investigating a car bomb explosion in Derry have arrested a further two men.
The men, aged 34 and 42 years, were arrested in the city on Sunday evening.
Two men aged in their 20s were arrested in the early hours of Sunday morning.
Earlier (5.35pm): Police have released dramatic footage showing the moment a car bomb exploded outside Derry Courthouse.
The footage shows a group of young people walking past the vehicle just minutes before it exploded.
The PSNI released four clips via Twitter on Sunday following the blast on Saturday evening.
Assistant chief constable Mark Hamilton described the bomb as a “crude device” and the attack as “unbelievably reckless”.
Dissident republican group the New IRA is suspected to be behind the attack.
The first clip shows the Ford Fusion car, which was hijacked from a pizza delivery driver earlier that evening, being driven to the courthouse in Bishop Street.
The second clip shows a figure running away from the vehicle after parking it.
Police release CCTV footage of a group of young people walking past car outside courthouse on Bishops Street in Derry/Londonderry last night. pic.twitter.com/DL0hnfX8Xj— PSNI DC&S District (@PSNIDCSDistrict) January 20, 2019
The third shows a large group of young people walking past the vehicle, as well as another car driving past.
The final clip shows the dramatic moment the car exploded in a flash of light and debris being flung from the burning wreckage.
Mr Hamilton described the bomb attack as a “very significant attempt to kill people”.
“Fortunately it didn’t kill anybody and fortunately it didn’t cause widespread damage but clearly it was a very significant attempt to kill people in the local community,” he said.
He added they got everyone evacuated “just in time”.
“The bomb detonated just as we were leaving the area and it is only by good grace that local people were not killed,” he added.
Earlier (12:06pm): Two men arrested in connection with Derry car bomb attack
Two men in their 20s have been arrested in connection with a car bomb attack in Derry last night, the police have confirmed.
The attack came shortly after 8pm last night when a vehicle exploded on Bishop Street.
Police and army bomb experts remain at the scene.
PSNI Assistant Chief Constable Mark Hamilton said the New IRA was their main line of inquiry.
"Our main line of inquiry is against the New IRA," he said.
"The New IRA, like most dissident republican groups in Northern Ireland, is small, largely unrepresentative, and determined to drag people back to somewhere they don't want to be."
Two men, both in their twenties, have been arrested by detectives investigating explosion outside Bishop Street courthouse in Derry/Londonderry.— PSNI (@PoliceServiceNI) January 20, 2019
Mr Hamilton detailed how a pizza delivery driver was hijacked on Saturday evening, and his car packed with explosives before being left outside the court house on Bishop Street in Derry.
“We believe that some time after 6pm last night (Saturday), in the Quarry Street area of Derry, a pizza delivery driver was hijacked,” he said.
“He had his Ford Fusion car taken off him by at least two armed men.
“Between then and 7.23pm, a bomb was put in that car, driven at least half a mile to outside the court house on Bishop Street and around three minutes later a phone call went in to the Samaritans in West Midlands in England, which was then passed to West Midlands Police who then contacted us.
“In the intervening minutes we had already found the car and started to evacuate the area. At around 8.09pm, the bomb detonated.
“Fortunately it didn’t kill anybody and fortunately it didn’t cause widespread damage, but clearly it was a very significant attempt to kill people in the local community.”
He added they got everyone evacuated “just in time”.
“The bomb detonated just as we were leaving the area and it is only by good grace that local people were not killed,” he said.
Those evacuated from nearby buildings included hundreds of hotel guests, 150 people from the Masonic Hall and a large number of children from a church youth club.
Earlier fears of a secondary device have been ruled out.
No-one was injured in the attack.
Earlier, the PSNI said the attack was "unbelievably reckless" and said they believe that the car had been "hijacked" in the city a short time before the explosion.
Politicians have condemned the incident, with Tanaiste Simon Coveney saying that there is "no place and no justification possible for such acts of terror".
Derry's SDLP mayor John Boyle challenged those responsible on what the aim was, while DUP leader Arlene Foster said it was a "pointless act of terror" which "must be condemned in the strongest terms".
Northern Ireland Secretary of State Karen Bradley has said those behind the bomb attack will not prevail.
"This attempt to disrupt progress in Northern Ireland has rightly been met with utter condemnation from all parts of the community," she said.
"The small number of people responsible have absolutely nothing to offer Northern Ireland's future and will not prevail.
"Thank you to the PSNI and emergency services who are working so hard to keep people safe and secure."
Police have described last night's suspected car bomb attack outside a court in Derry as "unbelievably reckless".
The PSNI are investigating after the incident occurred in the city centre at 8.10pm last night.
Residents were evacuated and witnesses reported hearing a loud bang and seeing a large plume of smoke.
Assistant Chief Constable Mark Hamilton said today that a suspicious vehicle was spotted by police 15 minutes before the device exploded.
He said they believe that the car had been "hijacked" in the city a short time before the explosion.
Constable Hamilton said: "At around 7.55pm last night officers on patrol in Bishop Street spotted a suspicious vehicle and were making checks when, around five minutes later, information was received that a device had been left at the courthouse.
"We moved immediately to begin evacuating people from nearby buildings including hundreds of hotel guests, 150 people from the Masonic Hall and a large number of children from a church youth club.
"The device detonated at 8.10pm. At this stage it appears as though the vehicle used had been hijacked from a delivery driver in the Quarry Street a short time before the explosion.
"The people responsible for this attack have shown no regard for the community or local businesses. They care little about the damage to the area and the disruption they have caused.
"The area remains sealed off and church services at St Augustine's have been moved to St Peters and the cathedral services have been cancelled this morning as a result.
Constable Hamilton thanked the local community for its support and co-operation and asked anyone with information to contact police.
An investigation by the PSNI is underway after a suspected car bomb exploded in Derry last night.
The police, as well as firefighters and the ambulance service, sealed off the scene close to a courthouse on Bishop Street in the city.
Witnesses earlier said they heard a loud bang and saw a large plume of smoke, and local residents were evacuated.
There has been no reports of any casualties at this stage.
A photograph was tweeted from the PSNI Twitter account warned people to stay away from the area.
The tweet from the PSNI's Derry City and Strabane District account also warned members of the public that Bishop Street was closed, adding the words "suspected car bomb".
A further update was posted on the PSNI's Foyle Facebook page.
"As far as we know no one injured. There is another car we are not happy about. There are ongoing necessary evacuations."
Tánaiste Simon Coveney tweeted his disgust at the incident.
"I utterly condemn the car bomb terrorist attack in Derry this evening," he said.
I utterly condemn the car bomb terrorist attack in Derry this evening. There is no place and no justification possible for such acts of terror, which seek to drag Northern Ireland back to violence and conflict. @RTENewsNow https://t.co/sKUaXGCx5b— Simon Coveney (@simoncoveney) January 19, 2019
Derry's SDLP mayor John Boyle challenged those responsible on what the aim was.
He said: "I would actually like to ask the people responsible for this what it actually was that they thought they were going to achieve.
"It achieves nothing, it didn't achieve anything in the past, it didn't achieve anything right now.
"I have a question for them, what was this all about, because quite frankly this is not something that the people of this city wanted to see, it's not something they support, and as Mayor of Derry City and Strabane District Council, I have to say I feel it incumbent upon me to speak out on behalf of the vast, vast majority of people in Derry, Strabane and indeed across this island, this is not what we want.
"This is the past and it has to stay in the past. We don't want to see any more of it."
DUP leader Arlene Foster said it was a "pointless act of terror" which "must be condemned in the strongest terms".
"Only hurts the people of the City. Perpetrated by people with no regard for life," she said.
"Grateful to our emergency services for their swift actions which helped ensure there have been no fatalities or injuries."
Sinn Fein Foyle MP Elisha McCallion also condemned the explosion.
"This incident has shocked the local community," she said. "In particular, there are many elderly residents who live in the area who have been alarmed.
"Thankfully no-one appears to have been injured.
"Derry is a city moving forward and no-one wants this type of incident. It is not representative of the city.
"I would encourage anyone with information about this incident to bring it to the police."
PA & Digital Desk