Four people were arrested as police shut down voting facilities in Macau after activists ignored strong objections from Beijing and held an unofficial referendum on electoral reform, the event organiser said.

Residents of the Chinese gambling hub were told they could vote at several locations across the territory, as well as online, but poll organiser Jason Chao said those attempting to take part in person were stopped by police, who confiscated the tablet computers people used to cast their vote.

He said four activists were arrested after polling began in the morning.

"Our volunteers were subject to harassment by the police and four volunteers were taken away. Other stations were also forced to stop their activities," Chao told AFP.

"The authorities are using all means to disrupt our activity," he said.

"I can't imagine why the government has to clamp down on such a peaceful event. It's a very serious violation of human rights."

A spokesman for the Macau government said in a statement that the four were arrested for "disobeying" instructions from police.

The former Portuguese colony returned to Chinese rule in 1999, but 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.

The referendum was scheduled to run until August 31, when the enclave's new leader will be named by the 400-member committee.

It was intended to garner public support for reforms in the city of 550,000, but it is not clear if it will be able to continue or if authorities will also block the online voting system.

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

- 'Illegal and invalid' -

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

At 4:00 pm (0800 GMT), more than 1,900 people had voted, according to the event's official website.

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

China has hit back at the referendum, with a statement from Beijing's liaison office in the city saying that the enclave had "no authority" to hold the poll.

Activists hope the referendum turnout will exceed 10,000.

In May around 20,000 people marched against a bill to allow government ministers generous retirement packages, with many youngsters taking part hoping for greater accountability from their own government.

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

The winning proposal would allow the public or democratically elected lawmaker to nominate candidates.

Beijing has slammed the votes as "illegal and invalid".

China has promised to let residents of the former British colony 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.