Dozens of suspected scammers and romance fraudsters have been arrested after specialist financial investigators uncovered organised criminal networks operating across the UK and Nigeria.

In just one of a series of raids this month, police detained a man suspected of selling personal banking information online and enabling fraud through money laundering. Two Rolex watches and £3,000 in cash were seized from his home in the Midlands.

A total of 31 arrests have now taken place in Europe and Africa as part of Operation Seraphim.

BBC News has been given exclusive access to the City of London Police operation, which involves investigative work by the financial crime unit, the Cyber Defence Alliance (CDA), as well as coordination with the National Crime Agency and the Nigerian Police Force's cybercrime team.

Many of those arrested are suspected of romance fraud, where scammers often spend months nurturing a relationship with their targets before slowly encouraging them to hand over money.

Kirsty Guest, who was scammed out of £80,000, is just one of the hundreds of victims targeted by romance fraudsters over the last few years. After coming out of a difficult marriage, the florist from North Yorkshire met a man on a dating app, who called himself "Patrick". Over months, they struck up a romantic relationship.

"He built a rapport with me. There was no rush," she says.

"He explained about things in his childhood, we spoke about places where he'd lived, and he spoke about where I'd lived. So, there was no real sense that this was going the wrong way for me; there were no red flags."

She says she was excited when she got a message from him and when she could speak to him on the phone.

"It did make my day," she adds. Then, while he said he was on a business trip abroad, she had a worrying call.

"He said he'd been in an accident. I'm the kind of person who would help anyone. When he said that, I just wanted to help him. I wanted to make everything right."

"Patrick" gave her his bank details so she could help him with some financial transactions, and having a "nosey around", she could see he was wealthy.

But then he started asking for money to tide him over until he could recover from the accident.

The guest was sucked in, eventually transferring more than £80,000 of her own money to his account and those of his associates, almost certainly "money mules".

"These are not just gentlemen thieves, confidently swindling us of a few pennies here and there," says Craig Rice, who heads up the CDA. "They're ruining lives and creating misery."

Rice's team of experts - from intelligence services, law enforcement and finance - work with police to fight fraudsters who cost banks millions in repayments to customers who have been targeted.

Funded by 11 major banks, the unit has been operating in the shadows for close to a decade, but agreed to give BBC News exclusive access to its work earlier this year.

They hunt down the scammers online by identifying patterns in bank data and use automated software to search social media messaging boards.

We agreed not to reveal some of the details of their methods to prevent criminals from evading capture.

Instead of responding to victims' reports of fraud, the investigators go after organised networks of fraudsters who share scamming expertise or provide software and services for a range of illicit activities, including building fake bank websites, stealing bank account details, and laundering money.

Many criminals, hiding behind anonymous usernames, openly discuss their techniques on social media, sharing step-by-step guides on how to carry out scams.

Working out of a secure base in Canary Wharf, this little-known fraud squad pulls together dossiers of evidence which are then sent to police forces, who help track down suspects.

The strategy has so far resulted in around 500 arrests, and investigators say they can hit criminals like "Patrick" harder and faster.

Even after she went to the police, the fraudster called Guest to say he was sorry and that he loved her, drawing her back into his web of lies. Then he asked for more money.

The spell broke, and she threw her phone at the wall in fury.

"I was angry because I knew what I'd just done," she says. "I was angry because I knew that I'd lost everything. I was ashamed at who I'd become."

But what she stresses is that she was groomed by a man who had clearly become skilled in manipulating her.

"They are professional, and they are making massive volumes of money," she adds. "They're intelligent in what they're doing."

Detective Inspector Joel Gregory, from the Intelligence Development Team at the City of London Police, says the operation shows what can be achieved by working alongside the banking industry and international law enforcement.

"The scale and complexity of online fraud means no single organisation can tackle it alone, but through this joint effort we've been able to identify offenders, build intelligence and take coordinated action," he adds.

"This has been a significant team effort across multiple agencies, and it sends a clear message to those involved in fraud that we are actively monitoring these spaces and will take action to disrupt and pursue them."