Urban Britain has long witnessed a seemingly endless series of gangs of street toughs jockeying for petty power, intimidating passersby, engaging in criminal activity or simply hanging out on the corner out of sheer boredom.

From the Penny Mobs of Glasgow in the 18th century, to various Irish gangs across the country in the 19th century, to the racist white skinheads of the 20th century, the history of British gangs documents a colorful and ever-changing tale of violence, dispossession and ultimate assimilation.

Now, in the second decade of the 21st century, a new (but not entirely unexpected) phenomenon has emerged on Britain’s mean streets – the rise of radical Muslim youths determined to impose their views of public conduct and morality upon anyone who cross their paths.

More than 50 years after massive immigration to the United Kingdom from the Indian subcontinent, entire inner-city neighborhoods in metropolises like London, Birmingham, Manchester and elsewhere have become South Asian enclaves.

These immigrants and their second- (and now third-) generation descendants have dramatically altered the ethnic landscape in the United Kingdom. Some, particularly Indian Hindus and Sikhs, have generally fared well economically, while others, like Pakistanis and Bangladeshis, have lagged.

From the latter groups, young men, alienated from what they view as a hostile and discriminatory outside society and trapped in poverty, have turned inward, focusing on the faith of their ancestors in order to establish their own separate identity.

These measures have sometimes taken on a violent countenance, emboldened by jihadist movements in South Asia, the Middle East and North Africa, as well as by a view of Western Christian society as weak, corrupt, materialistic and doomed.

In recent weeks, British media have reported on so-called “Muslim Patrols” in parts of London in which Muslim youths harass, badger, bother and even physically assault anyone in their neighborhoods whom they believe are violating their extremist, fundamentalist version of Islam as embodied by Sharia law.

One band of London Muslims even filmed themselves on YouTube harassing and verbally abusing a homosexual man.

In that video one Muslim follows a man in Whitechapel, East London, and barks at him: "Get out of here you fag … Don't stay around here anymore," and condemned him for "walking through a Muslim area dressed like a fag.”

People drinking alcohol, women not dressed modestly and those perceived to be either prostitutes or homosexuals have faced severe reprimands and intimidation from these Muslim gangs. The patrols have also defaced advertising they find offensive.

In another video posted on YouTube, one youth Muslim proudly declared: “This is a Muslim area. … The Muslims have taken it upon themselves to command the good and forbid the evil and cover up these naked people.”

Such patrols have been witnessed in heavily immigrant East London boroughs like Waltham Forest and Tower Hamlets.

While British tabloid media have feasted on the sensationalist aspects of these Muslim patrols (i.e., a violent, dangerous inassimilable subsection of the population that poses a threat to mainstream Britons), some moderate Muslims have also sounded the alarm over these young vigilantes.

Maajid Nawaz, the chairman of the Quilliam Foundation, a British-based think tank that advocates against extremism, warned that these disorganized Muslim Patrols could become more sophisticated and dangerous, particularly if their ranks become increasingly filled by British-born jihadists with real combat experience in foreign nations returning home.

Nawaz, a British-born Pakistani and a former member of Hizb ut-Tahrir, a pan-Islamic global political party, explained that urban British Muslims are simply following the patterns already established by right-wing nationalists and militants across Europe.

“While this street-level problem festers across Europe, al-Qaeda and its affiliates are busy capitalizing on the chaos of the post-Arab Spring world,” he wrote in the Times of London.

“Syria, Libya, Mali and Somalia are being ravaged by jihadist outfits, and all of them are attracting European-born Islamists seeking the thrill of real combat. … Scores of young European-born Arabs and Somalis are following in the footsteps of British Pakistanis in traveling to lawless conflict zones. What happens when these men, schooled in the use of political violence in far-flung places, return to Britain?”

Comparing UK Muslim youth vigilantes to the far-right Golden Dawn party in Greece and even the infamous Brownshirts of Nazi Germany, Nawaz further declared: “The Muslim patrols could become a lot more dangerous and, perhaps willing to maim or kill if they are joined by battle-hardened jihadis.”

Not only Britain, but some other European countries with Muslim populations – including Denmark and Spain – have witnessed a similar rise of home-grown Islamic street militants and thugs.

“Something very worrying is spreading across Europe. Fascists and Islamist extremists alike are copying what Hitler's Brownshirts excelled at -- enforcing with threats and violence their version of the law in neighborhoods,” Nawaz wrote.

“I fear that the Muslim patrols are a sign of things to come. … The longer we stand by and watch the far Right and Islamists impose their dogma on our streets, the more the extremes will become mainstream for a rising new generation.”

Other Muslim moderates are also concerned.

Mohammed Shafiq, the chief executive of the Ramadhan Foundation, a Muslim youth organization that fosters interfaith peace, told British media: “We live in the UK and we are governed by UK law. There should be no mob rule. If people are involved in this behavior, then it is worrying, but it is an isolated incident.”

It is unclear how many Muslim Patrols actually exist and how much manpower they current enjoy. However, they could potentially have a large pool of potential candidates to choose from -- some 2 million Muslims now call Britain home, with about half of them in the London metropolitan area alone (accounting for one-eighth of the population).