When huge crowds took to Hong Kong's streets this week to voice anger at Beijing's ever-tightening grip, an unlikely group was marching in step: wealthy financiers fighting mainland influence over the city's markets and political life.

Joining rain-soaked protesters from all walks of life -- who by organisers' estimates numbered a record half-a-million -- was hedge fund manager Edward Chin.

Until this year, the unassuming 46-year-old had never joined a political organisation. But like many others in Hong Kong, he has grown increasingly worried about the influence of China's Communist rulers -- particularly Beijing's insistence that when Hong Kong's next leader is elected in 2017, the candidates must be drawn from a list of approved "patriots".

Chin joined the nascent pro-democracy movement now known as Occupy Central after meeting one of its leaders, legal scholar Benny Tai, in March.

"I did my due diligence on Benny," he joked.

Occupy Central, which has grown from a handful of activists into a powerful thorn in the side of Hong Kong's government, has threatened to paralyse the city's Central financial district with mass protests if its demands for electoral reforms are not met -- so it may seem surprising that bankers are signing up.

But for Chin and other financiers who have formed their own faction within the group, the pro-democracy movement presents an opportunity to address what they see as China's outsized influence on Hong Kong's crucial financial sector.

A battle was raging between "old Hong Kong tycoons and mainland newcomers", he said, over the upstarts' seemingly limitless but dubiously sourced supply of capital, and their predilection for appointing family members or those with "guanxi" -- contacts -- within the Communist Party and Chinese state-owned businesses.

Chin's group of 70 members are drawn from a wide range of banks and investment houses. While they represent just a fraction of Hong Kong's huge banking workforce, they have become a force to be reckoned with in the protest movement, using it to lobby against the growing clout of Chinese cash and nepotism in the sector.

"They are exceptional," chief organiser Benny Tai told AFP. "The most active group of their kind (within Occupy Central)."

Their cash has certainly come in useful -- the bankers have spent tens of thousands of dollars on campaign advertising and events in just a few short months.

An entirely separate budget and agenda allows them to concentrate on issues such as financial corruption and immigration policy that are not a priority for the "core" protesters.

"Whenever we need money, we just raise it in a few days between ourselves," Chin said. "We just sign the cheque."

- Nepotism fears -

Hong Kong is an Asian financial hub, and the city prides itself on low levels of corruption and the ability to attract international firms with favourable tax rates and pro-business policies.

But Lai Chong Au, a sales manager at a major mutual fund house and member of Occupy Central's finance professionals group, believes that is changing fast. Used to an economy "run on guanxi", mainland financiers -- flush with cash from China's extended boom -- expect the same when they do business with Hong Kong.

"These one-man band mainlanders -- they can raise one or two billion US (dollars) easy," she told AFP. "They get it from their father or friends of politicians or government."

While her firm is increasingly open to the opportunities the mainland offers, she voiced caution over losing links with established Western firms.

"(We need to) ensure all the international companies will feel comfortable staying in Hong Kong. If we are losing this, we lose our advantage," she added.

US investment banks including JP Morgan and Goldman Sachs have long been accused of hiring "princelings" - the offspring of the Chinese elite -- in their Hong Kong offices to boost their chances of business success on the mainland.

But Chin said this kind of alleged nepotism was not welcome. "We aren't running a 'sons and daughters programme' -- choosing them over someone with a Harvard MBA," he said.

- Fighting for fair play -

In some cases, tensions between bosses acquiescent to mainland power and employees who would rather temper this influence are now breaking into the open.

Last week the so-called "Big Four" international accounting firms issued a statement accusing Occupy Central of stoking instability in Hong Kong, only to see employees buy up advertising days later in a local newspaper to condemn the actions of their superiors.

"The integrity of the accounting and finance profession (rests) on them showing they do care and that they won't sell their conscience just to get business," said Chrisann Palm, a former Hong Kong finance professional who now lectures at an Australian university.

For Chin, Occupy Central is the starting point for something much bigger. "We're fighting for fair play and fair influence," he said. "The ultimate goal is not to occupy the streets -- and everyone knows it."