Irish Republican Army dissidents are suspected of being behind a car bombing outside a courthouse in the Northern Ireland city of Londonderry late Saturday in an attack authorities called "incredibly reckless."

Police in Northern Ireland said the bomb was "crude" and "very unstable," and was placed in a pizza delivery car that had been hijacked and driven to the courthouse, where it later exploded.

The Police Service of Northern Ireland released four video clips showing how the bomb exploded just minutes after a group of young people walked past the vehicle.

The footage shows the car being parked outside the courthouse before the male driver is seen getting out the car and running away.

Assistant Chief Constable Mark Hamilton said the "incredibly reckless" attack was a "very significant attempt to kill people" and that using such devices was an "act of madness," Sky News reported.

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"The people responsible for this attack have shown no regard for the community or local businesses," he said.

Two men in their 20s who have yet to be identified were arrested in the city in the early hours of Sunday morning, according to police.

Hamilton added that people in the area were evacuated "just in time" before the bomb detonated, after receiving a warning and evacuating the area. There were no reports of injuries.

"There was a very large boom and everything started to shake for just a second as the shock passed over," Michael Williams, who was in his house nearby, told Sky News. "Everything sort of shook, my desk was shaking, I was shaking, my chair was shaking."

The Police Service of Northern Ireland posted a photograph of a vehicle in flames and urged the public to stay away.

Hamilton said the "main line of inquiry" was that the bomb had been planted by a group known as the New IRA.

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More than 3,700 people died during decades of violence known as "The Troubles" before Northern Ireland's 1998 peace accord. Most militants have renounced violence, but small groups of IRA dissidents have carried out occasional bombings and shootings.

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Northern Ireland's power-sharing government has been suspended for two years because of a dispute between the main Protestant and Catholic political parties.

Uncertainty about the future of the Irish border after Brexit is adding to tensions, according to Sky News.

The Associated Press contributed to this report.