A scrawny millennial with gaunt features and a studious frown, Joshua Wong looks like he'd struggle to take on a large steak, let alone the might of Communist China.

Yet the bespectacled activist is the unlikely hero to a generation in Hong Kong, where he led a movement inspiring hundreds of thousands to join his cause for elections free from Beijing's interference.

At the age of just 17, he spearheaded mass blockades that brought parts of the Asian financial center to a standstill in 2014, sparked by restrictions from Beijing on how Hong Kong's next leader will be chosen.

Hailed as one of the world's most influential figures by Time, Fortune and Foreign Policy magazines, he is now the focus of an award-winning Netflix documentary due for release later this year.

"We hope people around the world recognize that social movements can make things happen. They can make things change," Wong, now 20, told AFP by telephone from Hong Kong.

"People may be depressed or downhearted with the political situation in their own country, but it's still optimistic to see hope and seek change by street activism."

"Joshua: Teenager vs. Superpower" tells the story of how Wong became one of China's most notorious dissidents after the mainland Communist Party backtracked on its promise of autonomy to Hong Kong.

- 'David and Goliath' -

Critics say the 79-minute documentary could not have picked a better moment, with political engagement piqued in the West as protesters take to the streets to decry the policies of new US leader Donald Trump.

"You have a lone teenager taking on China and it's one of the things that attracted me to the story. The odds don't get much bigger. Talk about David and Goliath," Los Angeles-based director Joe Piscatella said in an interview.

At the age of just 14, Wong campaigned successfully for Hong Kong to drop a pro-China "National Education" program, rallying a crowd of 120,000 to his cause.

He was one of the 78 people arrested in September 2014 during another giant pro-democracy protest after China reneged on a pledge made during the handover to give Hong Kongers the right to choose their next leader.

Umbrellas were used to shield activists from waves of police pepper spray, giving the nascent "Umbrella Movement" its banal yet iconic symbol of resistance.

Galvanized by Wong's passion, the Umbrella Movement made headlines around the world, but was ultimately unable to shake up Hong Kong politics after weeks of protest.

Wong continues to campaign under the banner of a new political party, Demosisto, for a referendum to determine who will rule Hong Kong after the "one party, two systems" principle codified in Chinese agreements with Britain expires in 30 years.

"I'm still hopeful for the young generation here. In Hong Kong, more young people may be legislators in the future. I would say that this is just a starting point," Wong said.

- 'That's my life' -

Born to middle class Christian parents Grace and Roger Wong in 1996, Wong began his life of activism at age 13 with a protest against plans for a high-speed rail link between Hong Kong and the mainland.

It was here that Piscatella's producer, documentary filmmaker Matthew Torne, first encountered Wong and, seeing something extraordinary in the youngster, started his camera rolling.

"The first time I met Joshua, I was in awe... He's kind of a conundrum in that, when he walks into a room, he's not somebody you notice right away," Piscatella said.

"You give him a microphone and a bullhorn and there's a change in him where suddenly he just becomes this other person where he's passionate and has this ability to connect with a large group of people."

"Joshua: Teenager vs. Superpower" was picked up by Netflix and awarded the audience prize at this year's Sundance Film Festival, where Wong attended screenings, describing the support for the film as "unbelievable."

Since the end of the Umbrella Movement, Wong has been denied entry into Malaysia and Thailand, attacked in the street and abused by pro-China protesters in Taiwan. But he takes it all in his stride.

"That's my life," he shrugs, describing the drawbacks of his high profile, with a quiet insouciance, as "inconvenient" and vowing to fight on.

"We didn't win in the last battle," he said, "but I'm still optimistic for winning in the final war."