Video footage of the moment a car burst into flames outside a courthouse in Northern Ireland in a suspected bomb attack has emerged.

In the footage released by the police, the stolen car can be seen parked directly outside the building with its rear lights on before huge flames shoot into the sky, sending parts of the vehicle flying through the air.

The vehicle, stolen from a pizza delivery driver by two armed men, exploded shortly after 8pm on Saturday.

Two men in their 20s were arrested in the city in the early hours of Sunday by detectives investigating the blast.

Another two men, aged 34 and 42, were arrested in Derry on Sunday evening.

Dissident republican group the New IRA is suspected of being behind the bomb attack.

Assistant Chief Constable Mark Hamilton said on Sunday: “Our main line of inquiry is against the New IRA.

“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.”

He described how a pizza delivery driver was hijacked in the Quarry Street area of Derry some time after 6pm on Saturday before his Ford Fusion was packed with explosives and left outside the courthouse.

Earlier fears of a second device have been ruled out.

No one was injured in the attack, which has been condemned by leaders and legislators across the political spectrum.

"The small number of people responsible have absolutely nothing to offer Northern Ireland's future and will not prevail," Karen Bradley, Secretary of State for Northern Ireland, said.

"This is intolerable violence and we want to look forward and build a peaceful future for all in Northern Ireland."

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Ireland's leader Leo Varadkar condemned the attack as "an appalling, reckless and cynical act of terror".

Derry was a flashpoint for violence during the Troubles in the 1960s and 1970s.

"This is the past and it has to stay in the past," said John Boyle, the city's mayor.

"We don't want to see any more of it."