Police in Londonderry have confirmed that two of the three vehicles used in security alerts across the city on Monday were hijacked at gunpoint.

The third vehicle was hijacked by masked men who did not appear to be armed.

Speaking on Tuesday, after three separate security alerts in the town saw hundreds of residents evacuated from their homes, Derry’s top police officer Gordon McCalmont condemned those involved.

“Two out of the three – there has been mention of guns being involved,” he said.

“Those drivers were going out to their work yesterday, going to do a day’s work and support their families, and I’m sure not for one minute did they think they would be confronted by armed masked men.

“It’s madness, that is the reality of it.”

Royal Mail said on Tuesday that following advice of the Police Service of Northern Ireland (PSNI), their workers would not be attending certain areas of the city after one of its vans was hijacked.

The PSNI say its main line of inquiry is that Saturday’s bomb and Monday’s spate of alerts were conducted by the New IRA, an amalgamation of dissident republican factions in Derry, who are allegedly responsible for the riots in the Bogside area of the city during the summer.

Superintendent McCalmont added that he believed Monday night’s security alerts were planted to impede the investigation into Saturday evening’s car bomb attack in Bishop Street.

In the first incident, three men reportedly hijacked a white Transit van in the Circular Road area at around 11.30am before throwing an object in the back and abandoning the vehicle.

Army bomb disposal experts carried out a controlled explosion on the van.

Just over two hours later, at 1.45pm, police received a report that four masked men – one allegedly armed with a gun – had hijacked a postal delivery van on Southway.

Police said the two occupants of the van were ordered to drive to Lonemoor Road and leave it there.

Both incidents prompted sizeable security alerts, with nearby residents evacuated.

The third incident saw another abandoned vehicle in the city cause panic on Monday night.

Police attended Northland Road after an Asda delivery van was left parked across the road, stopping traffic in front of St Mary’s secondary school.

Elderly residents were evacuated from their homes as police attempted to secure the area.

Mr McCalmont added: “The support we have had has been absolutely fantastic, we’ve had residents thanking ATO and officers when being evacuated from their homes – we’re working hard as a community, political, church and otherwise.

“I’d like to repeat that Derry is very much open for business today.”

Four men arrested on suspicion of the Bishop Street car bombing have been released.

The men, aged 42, 34 and two aged 21, were arrested on Sunday and released unconditionally.

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

The PSNI has refuted criticism that the force rushed into the arrests in order to calm a worried public.

“Everything we have to do is lawful and necessary – that criticism is unwarranted,” Mr McCalmont added.

No new check points will be set up in the city as the PSNI does not want to add to the disruption already caused by “terrorists”.