China has dismissed a proposed referendum on democracy in Macau, a poll that would follow in the footsteps of a similar informal vote in Hong Kong.

Three activist groups said on Monday that they would stage a referendum among Macau's 600,000 residents to coincide with the widely expected re-election by a local council of local leader Fernando Chui in August.

"An administrative region has no authority to establish a system of referendum or organise any activity relating to a referendum," China's Liaison Office, which oversees affairs in Macau, said in a statement posted on local media websites.

The statement said the office supported the position of Macau's authorities, who are subservient to Beijing.

China denounced the June poll in Hong Kong, underscored by a march by hundreds of thousands of protesters demanding the right to freely elect their local leader in 2017. Five student leaders were arrested after a later sit-in.

Hong Kong is a former British colony which returned to Chinese rule in 1997. Shortly after, Macau which was taken over by the Portuguese was returned to China as well.

Soaring inequality

Both regions are partly autonomous and are allowed freedoms not permitted in mainland China.

An official body of 400 elects Macau's leader, similar to Hong Kong where a small committee of largely pro-Beijing loyalists chooses who gets on the ballot, effectively rendering the ability to vote meaningless.

Macau's government earlier said the referendum "had no constitutional legal basis, does not have any legal basis, is illegal and invalid".

Residents of Macau, home to 35 casinos and the only place in China where casino gambling is legal, adopted a passive attitude to politics during years of rapid economic growth.

But over the past year, soaring inequality and deteriorating quality of life sparked an unprecedented 20,000-strong rally to denounce a bill providing lavish perks for senior civil servants. The legislation was withdrawn.

Chinese authorities, who consistently crack down on dissent on the mainland, reasserted their authority over Hong Kong in an authoritative report issued before the vote there, a move that triggered fears of future intervention.