The boy, held from Walthamstow in east London, is being interrogated over the videos posted on YouTube featuring a group of men harassing passers-by for being gay, drinking alcohol or wearing mini-skirts on January 12 and 13, Scotland Yard said.
It is the latest in a series of arrests made since the Metropolitan Police launched an investigation after videos posted online showed numerous cases of abuse and harassment.
In one clip entitled "Muslim vigilantes in London harass and taunt gay male", they are heard shouting homophobic abuse and in the second video they are seen telling another man "no drink in this area, it's a Muslim area" before ordering him to pour away his alcohol.
Police have urged the man targeted with homophobic abuse to come forward as "a crucial witness".
Detective chief inspector Wendy Morgan said the Met takes such "homophobic behaviour" very seriously.
"This man is a crucial witness in the investigation and would encourage him and anyone else with information relating to this incident to make contact in confidence as soon as possible," she said.
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The "Muslims enforcing Sharia Law on the streets of London" video showed further verbal attacks, including a woman in a mini-skirt being asked not to "dress like this in a Muslim area" and party-goers being moved away from a mosque as "alcohol is evil".
All of those arrested have been held on suspicion of grievous bodily harm (GBH) and public order offences.
On January 20, a 22-year-old was held in Acton, west London, and released on bail until March 12.
The following day, a 19-year-old was arrested after he went to an east London police station and was bailed until March 11.
On January 24, two men aged 25 and 29 were arrested and remain in custody at police stations in east London.