TEHRAN, Young Journalists Club (YJC) – Created only in 2015, the leftwing activist group already has 40,000 members and is widely respected for its campaigning skills -- even by Prime Minister Theresa May's Conservatives.
On a recent Saturday, some 200 Momentum activists descended on the borough of Westminster in a bid to convince voters to back the most radical Labour Party in decades.
One of them, Ash Patterson, admitted he was nervous about preaching politics on people's doorsteps as he began canvassing the area but said it was all about "speaking from the heart".
Andrea Mann, a local Labour candidate, said Momentum was helping to bring "people into politics who have never been involved before".
"They bring young and older people... there is a real mixture among Momentum -- the more the merrier," she said.
The movement is dwarfed by Labour's membership numbers, which stand at nearly 600,000 and have spiked since Corbyn became leader of the party in 2015.
But it is growing fast at a pace of 500 new members a week.
"They punch above their weight," said Pancho Lewis, another Labour candidate in Westminster.
"Their influence is felt a lot -- they are very active, they are hard-working campaigners, and they are very clever about how they campaign."
The movement is ever-present on social media.
It has also developed digital apps and websites -- such as "My Nearest Marginal" -- to target constituencies earmarked as potentially competitive.
The app is used as part of the group's "unseat" campaigns targeting high-profile Conservative candidates.
"It is one of those ideas when you look back, you think why didn't we have this before," said Paddy Wilson, a 23-year-old recently enlisted activist, about the application.
"You just put in your postcode and it tells you where you nearest marginals is and you can go there and help."
The tool brought Wilson to Westminster, where the council has been ruled by Conservatives for decades but Momentum believes is potentially winnable.
"It is a big challenge but it is still possible, and that's partly because of the number of people who went to knock on doors," said Owen Jones, a leftwing political commentator who helped found Momentum.
Jones said he wanted the group to become "a mass movement" that could help propel Labour to power at the national level.
"A lot of the people here today had never knocked on a door before," Jones told AFP.
"We want to turn them into activists and that means when a general election is called in this country, we will have the biggest army of volunteers in the history of British democracy."