Jim Gamble, former head of the Child Exploitation and Online Protection Centre, says this is a "shocking case".

"The sheer scale of it and the horrific nature of the harm inflicted on these young girls makes it one of the worst I've ever seen," he tells BBC News.

"Don't watch this and think this happens very rarely," he says, adding that there are offenders out there using this (online) environment to target children.

His advice for parents is to educate themselves about what's happening online, talk about it to their children - about what to look for in a case where someone sexually exploiting a child is using the catfishing technique, which is pretending to be someone other than who they are.

Luke Sproule
BBC News NI at Belfast Crown Court

The judge says there are young girls and their siblings around the world whose “childhoods have been scarred by this defendant”.

In some cases, he adds, their parents or guardians might not even know yet of what had been done to them.

Luke Sproule
BBC News NI at Belfast Crown Court

McCartney is sitting in the dock with his head bowed as the judge begins to set out details of his offending.

Luke Sproule
BBC News NI at Belfast Crown Court

The judge is now setting out the maximum sentences which McCartney could face for the various charges he has pleaded guilty to.

He adds that the sentence passed will reflect the fact that guilty pleas avoided the need for a trial.

McCartney has pleaded guilty to 185 charges of child sexual abuse crimes and blackmail, along with one manslaughter charge.

An earlier court heard that McCartney had been selling indecent images on a fraudulent account, through which he earned £700 in three weeks.

He had previously told police that he had a gambling addiction and was £1,000 in debt.

McCartney also claimed that he had been the victim of catfishing in his teens, but prosecutors said there was no evidence to support that claim.

In mitigation, defence barrister Greg Berry KC said it was "not hyperbole to describe this case as quite horrific” but he added that McCartney offered genuine remorse.

He said that the fact that McCartney said that he himself had been catfished was not an excuse but that it had "shaped his behaviour”.

He added: "What we are dealing with now is someone who is now 26. He has been in custody now for over five years and has clearly had a chance to reflect on the complete horror that his actions caused."

Luke Sproule
BBC News NI at Belfast Crown Court

The judge says McCartney has set out some mitigating factors for his crimes - but Mr Justice O'Hara describes them as “few in number and limited in nature”.

Luke Sproule
BBC News NI at Belfast Crown Court

The judge says the full details of McCartney’s crimes will not be set out again today as that has already happened in court.

He describes them as crimes of “sadism and depravity”.

Mr Justice O'Hara is presiding over the case at Belfast Crown Court and is due to pass sentence this afternoon.

During an earlier hearing, some of McCartney's messages to the children were read to the court.

Others were provided to the judge in a pack, which he said he read with "great difficulty".

"It’s excruciating really," he said.

“We’re into new territory here really.”

The judge said some of the offences can carry a life sentence.

Luke Sproule
BBC News NI at Belfast Crown Court

McCartney has been brought into the dock in handcuffs.

He is wearing a grey tracksuit top and is in a grey t-shirt and is staring at his feet, flanked by two custody officers.

Mr Justice O’Hara has taken his seat in the court and is about to begin his sentencing remarks.

Image source, Getty Images

Although investigators believe as many as 3,500 children were targeted; this case focused on 70 of them in order to provide a manageable caseload for the court.

In 2019, it was confirmed that the Police Service of Northern Ireland had set up a "dedicated team" to specifically examine the McCartney case, due to the number of potential victims.

Identifying the alleged victims had become a "mammoth task", an earlier court was told, with the investigation stretching as far as New Zealand.

The court heard that, initially, 12 computer devices were seized from McCartney following his arrest.

It was explained that these devices had the capacity to emulate other computers, giving the operator the ability to make contact with four times as many victims.

"Each emulator has 760 folders, and each of these folders identifies one victim," a prosecuting solicitor said.

"There is a small sample being brought forward for prosecution, which is just the tip of the iceberg."

In total 64 devices were found at his house in the rural Lissummon Road area outside Newry during four separate raids.

These devices contained thousands of images of abuse.

Luke Sproule
BBC News NI at Belfast Crown Court

We have taken our seats in court 13 where the media presence is so large journalists are having to sit in the empty jury seats.

There are a small number of members of the public in the public gallery.

McCartney targeted children across the world and today’s sentencing hearing has been scheduled for the afternoon so victims and their families in other time zones are able to watch it online.

Some have now begun joining the video link as we wait for sentencing to begin.

Alexander McCartney has spent more than five years on remand at Maghaberry Prison – leaving only for court appearances and further questioning by the police.

McCartney grew up five miles outside of Newry and just off the main road to Armagh city.

It’s about as rural as it gets. Farms, a church and a few businesses.

When he first appeared at Newry Magistrate’s Court in July 2019 he was just 21, with long hair and the wide-eyed look of someone surprised to be sitting where he was.

At that time, he was a final-year computer science student at Ulster University.

Image source, FACEBOOK

Speaking to BBC News NI, local sources described him as an “introverted and socially awkward” teenager.

Another person said: “He came across as a pleasant, affable, intelligent young man and you wonder: 'How did he get involved in this?'

"There is nothing extraordinary about him."

We now know he was involved in the severe sexual exploitation of young girls aged 10 to 16 on social media, mostly Snapchat.

At an earlier hearing, the prosecution said he committed the crimes for his own sexual gratification.

Luke Sproule
BBC News NI at Belfast Crown Court

There is a large media presence in the waiting area outside court 13 in Belfast Crown Court.

We are expecting the sentencing to begin at about 14:00 BST but it could be some time before the judge actually spells out what sentence he is imposing on McCartney.

It was back in 2010 when the word "catfish" took on a new meaning - someone who uses a fake identity online to target someone, usually for abuse or fraud.

The phrase came from documentary maker Nev Schulman, who fell in love with a 19-year-old girl online, only to find out she was actually a housewife using fake photos and a false story.

Catfishing is not a specific offence in the UK, but criminal elements of the activity could be covered by different parts of the law.

McCartney is scheduled to appear before Belfast Crown Court for sentencing for 185 charges of child sexual abuse crimes and blackmail, along with one manslaughter charge.

The charges cover a period from 2013 to 2019, with victims being identified in both New Zealand and the USA.

The time of sentencing, 14:00 BST, was chosen to accommodate victims and their families in different countries, who will be watching via video link.

Following sentencing, we’re expecting to hear from the Police Service of Northern Ireland as they hold a news conference outside the courthouse.

Peter Coulter
BBC News NI

Image source, Facebook

Alexander McCartney is one of the world's most prolific online child abusers.

From behind a computer screen at his home in Northern Ireland, he brought fear and devastation to the lives of thousands of children across the world.

Police believe as many as 3,500 children were targeted by McCartney, on his 64 devices. He pretended to be a young girl, to lure his victims, many who were struggling with their sexual identity, into sending intimate photos.

Once he had the photos he would blackmail them, not for money, but for more extreme photographs and often force them to include younger siblings in the abuse.

The court heard the harm McCartney caused was "unquantifiable" and he "degraded and humiliated" victims for his own sexual gratification.

Many of his child victims have never been identified, but all their lives have been changed forever.

In the next hour, at a court in Belfast, Justice O'Hara will begin to sentence McCartney, who pleaded guilty to 185 charges including one of manslaughter.

We'll bring you live updates from the court and reaction after he has been sentenced.