From Neighbourhood Watch to the arrest of criminals caught in the act, all over Europe ordinary citizens are helping police do their job.

In some counties, it is simply tolerated, In others, these vigilante groups are actively encouraged. But what are the potential risks of these vigilantes taking the law into their own hands?

In Wembley, north west London, Patrick and Cleveland are in front of the computer, surfing the Internet on messaging platforms for young people aged between 10 and 19-years-old

The two men have created a false account and are posing as a 12-year-old girl. It's not long before they are contacted by a man who starts to discuss having possible sexual relations with the youngster.

Like thousands of other British citizens, Patrick and Cleveland are self-proclaimed paedophile hunters.

"Very quickly he gave us his phone number for the contact on WhatsApp and very soon he started to talk to us about sex," Patrick explains. "He sent us very shocking pictures of him totally naked and he sent several pictures of his penis."

A previous virtual contact now turns into a real life hunt.

The practice of vigilante groups came from the United States and has spread to Europe especially in countries in the north.

In Scandinavia, The Netherlands, Germany and the UK, vigilante groups number more than three million citizens. But what are the potential risks and do these vigilantes jeopardise the presumption of innocence?

Vigilante groups in The Netherlands

In The Hague, in The Netherlands, ordinary citizens - both men and women - volunteer to patrol the city twice a week in their spare time.

Jan Overduin is one of these vigilantes.

"In the evening when it is dark, we check that the public lighting works well that sort of thing," he says.

By using a mobile application especially created for these vigilante groups, they can alert the local authorities in real time about all kinds of damage or anti-social behaviour. By patrolling the streets looking for crimes, these citizens intervene sometimes before the police.

In The Netherlands, crime figures have been falling steadily since 2012. But the number of prevention and self-defense groups in the country is on the increase.

Sociologist Shanna Mehlbaum has studied the phenomenon and says contrary to popular belief, these groups are more prevelant in areas with existing low crime rates.

"It would be much more logical to have these kinds of neighbourhood watch groups in unsafe areas. But this is not often the case." she says. "People feel the need to feel safer when they are in very secure neighborhoods. The conclusion of our research is that these groups have no real influence on the crime rate in their neighborhood."

The Netherlands, like other countries, has encouraged the emergence of these citizens' movements without setting out any guidelines. This tolerance is denounced by police unions.

"Sometimes there is an outburst of violence and of course it is illegal. So we are trying to make people aware of the problem," says Gerrit Van De Kamp, president of the ACP police union. "These groups have limits and it is necessary to insist and define a legal framework on what these groups can or cannot do . Otherwise you may become a suspect yourself."

Fear of other people and fear of foreigners is the motivating force behind many vigilante groups in Europe. And politics is never far away

German neo-Nazi movement

In Germany, the NPD, close to the neo-Nazi movement, created controversy with a video which makes clear the group's objective is to secure the city of Amberg against migrant attacks.

Marc Schuilenburg is a criminologist. He believes vigilante groups like the one in Amburg can lead to discrimination, stigmatisation and ethnic profiling.

"(They target) to say it broadly black men, men with long beards, women with burkas, etc," he explains. "All things and people who deviate from the norm in their neighborhood become suspicious."

This completely voluntary workforce has become essential for many European nations lacking a security budget. And one country in particular has taken this way of thinking one step further giving the public the right to carry out their own investigations.

A Citizen's Arrest authorises the public to apprehend someone in the event of a noticeable offence or if the intention to commit a crime seems obvious.

Paedophile hunt

Back in Wembley, Patrick and Cleveland are continuing their hunt. They now have a number of details about the suspect including his name, date of birth and profession. They intend to go to the suspect's house to arrest the alleged sexual predator.

But the suspect is not at home and it takes them a further eight days before they track him down in the street. They confront the man with all the messages and photos he sent to the person he believed was a 12-year-old girl. This confrontation is broadcast live on the internet.

In front of 2,400 people, the man's identity is revealed and the presumption of innocence clearly flouted. Patrick and Cleveland now call the police.

Patrick Fripps has delivered more than 600 alleged sexual predators to the authorities.

The police believe this practice can help to put dangerous criminals out of action, but it is far from an ideal solution.

"These groups of paedophile hunters cause us more problems than they help solve," explains Dan Vajzovic of the Cambridge Police. "Sometimes these groups become illegal by hitting people, hiding evidence, or committing extortion or blackmail."