Two Hong Kong expatriates Wednesday ran a 102 kilometre (63 mile) ultramarathon following a course in the shape of a giant umbrella to support pro-democracy protests in the southern Chinese city.

The pair started at 4am and ran for over 18 hours on a path that took them from the city's mountainous country trails to the main protest site opposite the government's headquarters.

John Ellis and Andrew Dawson, both seasoned ultramarathon runners, said they wanted to show solidarity for those camped out on the streets demanding full democracy for the former British colony.

"We deliberately wanted to make it long because it's symbolic of what we think will probably be a fairly long struggle ahead," Ellis, a 36-year-old Australian who works in investment, told AFP hours before the pair began their run.

The protests have been dubbed the "Umbrella Movement" following the creative ways demonstrators have used them to shelter from the heat, torrential rain, tear gas and pepper spray and police batons.

Ellis and Dawson wanted to find a route that would look like the protest movement's symbol if superimposed on a map of the city.

Their race began at the Upper Shing Mun reservoir in Hong Kong's New Territories, a hilly rural region close to the mainland and far removed from the concrete jungle which the Asian financial hub is better known for.

Their route took them on a loop of the outlying New Territories before turning south towards the city's densely packed Kowloon district and then across the harbour to Hong Kong Island.

They passed through the first protest camp in the Mongkok district late afternoon before ending their run at the main protest site in Admiralty opposite the government headquarters.

The two Australians were joined by dozens of runners by the final stages of their ultramarathon.

- 'This is where it gets tough' -

Shortly after 3pm Ellis posted a picture on Facebook of his GPS watch showing the pair had run 74.3 kilometres so far and were around two hours behind schedule.

"This is where it gets tough. The legs hurt, your feet scream and your brain starts trying to negotiate with yourself," he wrote.

Ultra races, defined as any distance beyond the 42.2-kilometre marathon, are becoming increasingly popular around the world.

And with its 300-kilometre network of trails criss-crossing rocky terrain, exposed peaks, bays and reservoirs all close to the city, Hong Kong is an ideal venue for such races.

"It's not going to be easy. There's going to be lots of moments where we will feel like giving up or it just gets too hard but we want to push through and see it to the end. And hopefully that symbolism is the same for this democratic movement in Hong Kong," Ellis said before the run.

Many of Hong Kong's largely wealthy and often transient expat community have been unsure whether to involve themselves openly in the city's democracy movement.

Some fear open support might risk feeding China's allegations that "foreign forces" are behind the protests. Others feel the city's internal politics are not their battle.

But Ellis said he felt it was important for the foreign community to stand up for Hong Kong's democratic future.

"I've met those expats over here who believe it's not their fight. An expat can always leave. But if there's a chance to make Hong Kong better, and I think having a government that truly represents the people... then that's something I really want to get involved in," he said.