Beijing has slammed an unofficial referendum on electoral reform to be held in the gambling mecca of Macau, local media said Thursday, 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.

A statement from Beijing's liaison office in Macau said the enclave had "no authority" to hold the poll, which organisers said was part of a fight to establish a democratic system.

"An administrative region has no authority to establish a system of referendum or organise any activity relating to referendum," a report in the Macau Daily Times cited the statement as saying Thursday.

Macau's poll is slated to run between August 24 and August 30 -- just ahead of the naming of the enclave's new leader on August 31 -- and activists are hoping for a turnout of 10,000 in the city of 550,000.

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.

Macau government spokesperson Alexis Tam described the poll as "illegal" and "invalid", the paper said, in echoes of Beijing's criticism of Hong Kong's recent unofficial referendum.

Hong Kong Chief Executive Leung Chun-ying also described his city's poll as a "so-called referendum" with no legal foundation.

In May around 20,000 people in Macau 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," poll organiser Jason Chao told AFP on Tuesday.

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.

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, angering Beijing.

Each of the Hong Kong poll's proposals included an element of public choice.

The referendum was followed by a major pro-democracy march on July 1.