Activists said Wednesday they expect more than half a million people to take to the streets on Hong Kong's annual protest day in what would be the biggest march since Britain handed the city back to China.

The predictions for the largest July 1 protest since the handover in 1997 came after 740,000 people voted in an unofficial poll organised by pro-democracy activists.

"We expect the numbers of participants in the rally might actually exceed the numbers of 2003," Johnson Yeung, head of the Civil Human Rights Front, which organises the annual march, told AFP.

That march saw 500,000 take to the streets in protest at a security bill, which was then shelved.

"If the number exceeds that, this will be the largest (since the handover), said Yeung.

"Hong Kong people have started to think about their future and they fear that future may be oppressed by the central government," he added.

The July 1 rally in the semi-autonomous Chinese city sees citizens protest on multiple issues, usually dominated by calls for democratic reform.

The rally will come two days after the close of the unofficial referendum organised by pro-democracy group Occupy Central.

The referendum, which has angered Beijing, puts forward proposals that would give the public a say over the choice of candidates for the city's leader.

Hong Kong's chief executive is currently appointed by a 1,200-strong pro-Beijing committee.

Residents will be able to vote for chief executive in 2017, but China has rejected the idea of giving them a say in who can stand for the post. Many pro-democrats fear Beijing will hand-pick sympathetic candidates.

- A 'landmark' year -

Anger in the city has grown following a white paper published by Beijing earlier this month that reasserts China's control over Hong Kong.

The city was handed back to China by former colonial power Britain on July 1, 1997 under a "one country, two systems" agreement, which allows residents civil liberties not seen on the mainland, including free speech and the right to protest.

But there is increasing concern those freedoms are being eroded.

"I would call it a landmark year because the release of the white paper has aroused the fears of many people over whether 'one country, two systems' has come to an end," said Ivan Choi, a political lecturer at the Chinese University of Hong Kong.

"Hong Kong society has a strong sentiment against the government, and the white paper will provoke more people to come out (for the July 1 rally)," he said.

Three leading Taiwanese activists, one of whom was planning to attend the march, said Wednesday that Hong Kong had turned down their visa applications for the visit.

"We can't think of any reason to deny our entry to Hong Kong other than political reasons," Lin Fei-fan, one of the student leaders who occupied Taiwan's parliament in March, said in a statement.

Hong Kong lawmaker and veteran protester Leung Kwok-hung, known as "Long Hair", will also not be present at this year's rally after he was jailed earlier this month for disorderly behaviour.

Occupy Central is planning to paralyse Hong Kong's financial district with thousands of protesters at the end of the year if officials do not allow public nomination for the city's leader.