A series of security alerts in Londonderry on Monday were designed to "frustrate" an investigation into a bomb that exploded in the city centre, police have said.

Three alerts - triggered when two vehicles were hijacked by masked men and a delivery van abandoned - ended in the early hours of Tuesday morning.

Police said residents have returned to their homes and the alerts were hoaxes.

The disruption came less than 48 hours after a bomb exploded on Saturday.

The bombing happened outside the Bishop Street Courthouse. The courthouse has since reopened.

Police have said the bomb may have been carried out by dissident republican group the New IRA.

Members of the DUP are to meet Northern Ireland's chief constable on Tuesday morning to discuss recent incidents in the city and the ongoing dissident republican threat.

In addition to the three security alerts in Derry, a controlled explosion was carried out on a suspicious object in north Belfast on Monday night.

Police have released four out of five men held over Saturday's bombing.

A 50-year-old man arrested on Monday remains in police custody.

The PSNI's district commander in Londonderry, Supt Gordon McCalmont, told BBC Radio Foyle it was his understanding that the those behind the hoax alerts "were trying to frustrate our investigation into the initial bombing incident on Saturday".

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He added that the alerts were meant to "cause maximum disruption to the community".

Army bomb disposal officers carried out a controlled explosion in Creggan's Circular Road after a vehicle was hijacked by three masked men at 11:30 GMT on Monday.

The second alert on Southway, also in Creggan, followed reports of a vehicle being hijacked by four masked men at 13:45.

In a third incident, police responded to a report of an abandoned lorry on the Northland Road close to the Glenbank Road junction and St Mary's College.

"Three controlled explosions were carried out in total on Monday," Supt McCalmont said.

"There was certainly a mention of guns being shown by masked men to these people in two of the hijacking incidents.

"Only those responsible can explain why they are inflicting fear on our community."

'Stretched and under pressure'

Supt McCalmont also said no devices were found in any of the three vehicles.

"I have full organisational support and our focus is on keeping people safe. We will have proportionate operations in place.

"As a district commander, I am extremely proud of my colleagues over the past couple of days.

"We dealt with Bishop Street, we responded to 200 calls for burglaries, domestic abuse and vulnerable people.

"Although we were stretched and under pressure, we managed and delivered a service to the community."

St Mary's College, where the van was abandoned on the Northland Road, opened as normal on Tuesday morning.

Its principal, Marie Lindsay, told BBC Radio Foyle that additional support will be provided for students who may have been affected by the security alert.

A spokesperson for Northern Ireland's Housing Executive said one of their vans was hijacked and that a contractor they work with has withdrawn services in the Bishop Street, Brandywell, Creggan, Rosemount and Rossville areas until further notice.

A Royal Mail spokesperson also confirmed that one of their vehicles was at the centre of an alert in the Creggan area.

At the courthouse in Derry, scheduled jury trials have been put off until Wednesday.