Activists in the gambling hub of Macau have announced an unofficial referendum on electoral reform in the latest challenge to Beijing, after almost 800,000 turned out for a similar poll in Hong Kong.

The former Portuguese colony returned to Chinese rule in 1999 and has a separate legal system from the mainland. Like Hong Kong, Macau's leader is known as its chief executive and is chosen by a pro-Beijing electoral committee.

Three civil groups have joined forces to organise the poll which will run between August 24 and August 30 -- just ahead of the naming of the enclave's new leader on August 31.

"Our goal is to fight for a democratic electoral system and the first stage is to get the citizens informed of the election system," poll organiser Jason Chao told AFP.

"We hope that the referendum will be able to serve as a foundation for our fight for democracy in the future," he added.

"The referendum will give them (the voters) a chance to express their attitudes towards the system."

Questions include whether there should be universal suffrage for the 2019 chief executive elections and how confident voters are in sole candidate Fernando Chui, who has been in the position since 2009.

Chao said he hoped for a turnout of at least 10,000 -- Macau's population is around 550,000 -- with residents voting electronically and at polling stations.

In May around 20,000 people marched against a bill to allow government ministers generous retirement packages.

"Macau citizens were long considered apathetic to politics, but the big protest in May changed everything, with more young people and Macau citizens taking to the streets without fear," Chao said.

Hong Kong held an informal poll on democratic reform last month which saw more than 790,000 people vote over 10 days on how Hong Kong's next leader should be chosen.

China has promised to let Hong Kong residents elect the chief executive in 2017, but has ruled out giving voters a say in selecting candidates, prompting fears that only those sympathetic to Beijing will be allowed to stand.

Each of the poll's proposals included an element of public choice. Beijing slammed the vote as "illegal and invalid".

The Hong Kong referendum was followed by a pro-democracy march on July 1 which organisers said saw just over half a million people turn out for.

Police gave a lower figure of 98,600 taking part at the peak of the protest. More than 500 sit-in protesters were also arrested.