YouAintNoMuslimBruv became Britain's top trending Twitter hashtag Sunday after a passer-by shouted the dismissive remark at a knife-wielding man in a London Underground station.
Police are treating Saturday's attack at Leytonstone Tube station in east London as a "terrorist incident".
In video footage, an onlooker is heard shouting "You ain't no Muslim, bruv" as the 29-year-old suspect, reeling after being hit with an electric stun gun, is pinned down and handcuffed.
The dismissive retort was the top trending hashtag in both London and the whole United Kingdom.
Twitter user Arcanequaint called it "the most British response to a terrorist attack ever".
Another called Russburt wrote: "One man does more for community cohesion with one sentence than any government initiative."
Another called Muthla said: "Best trend I've seen -- YouAintNoMuslimBruv regarding horrific attack in Leytonstone. Gotta love London for the trend!"
Sociologist Craig Morris wrote: "One sentence pushes away extremists and unites everyone else. This is our stance now. Great."
Chris Lowndes said: "YouAintNoMuslimBruv is just perfect -- real, inclusive, undermines terrorist cause."
Twitter user Jackie Nn wrote: "youaintnomuslimbruv -- so proud to be a Londoner. True Muslims don't try and kill innocent bystanders. Peace to true Muslims."
The male voice heard on the amateur mobile phone footage said: "You ain't no Muslim, bruv! You're no Muslim, bruv! You ain't no Muslim!"
Onlookers also shouted "Drop it, you fool", referring to the knife, and "Who is this idiot?".
One eyewitness told the BBC that people barracked the suspect as police led him away and one person threw a bottle at him.
Twitter user Kellie O'Gara wrote: "Can we make Youaintnomuslimbruv the new battle cry against terrorism? No guns, no bombs, no threats. Just those five words."
Writer Will Black said: "This should be played on tannoys across Daesh (Islamic State) gang areas and dropped on posters from planes."