Hong Kong is braced for protests on Tuesday, which organisers expect will draw at least 500,000 people, potentially making it the region's largest demonstration in recent history.

The scale of the protests reflects frustration at Beijing's plans to choose Hong Kong's next chief executive. Nearly 800,000 residents – more than a fifth of the city's electorate – had cast ballots in an unofficial "referendum" as the polls closed on Sunday, according to the vote's organisers.

The pro-democratic protest group Occupy Central with Love and Peace (OCLP) announced that 798,000 Hong Kong residents had voted in its "civil referendum", an unofficial vote on how their next chief executive – the region's top official – should be elected. Beijing has bristled at the poll. One government official called it illegal and invalid, while China's state-run media has called it a farce and a folly. The referendum website has been barraged by highly advanced cyber-attacks.The region's activists remain unmoved. "The turnout [at this year's protest] will be a sort of signal, just as the referendum was a signal of the number of people who are satisfied with Beijing's method of choosing the chief executive," said Benny Tai Yiu-ting, an assistant law professor at Hong Kong university and one of the movement's leaders. "It takes just two minutes to vote. But attending the rally – that takes a lot of time and effort. That's evidence of stronger determination."

OCLP closed its final polling station at the university's student union building at 10pm local time on Sunday, amid intermittent rainstorms. Volunteers wearing green and yellow vests removed large cardboard ballot boxes, weaving through a scrum of local and foreign journalists pressed against the doors. Just outside the university, vestiges of recent counter-protests littered the pavements – scattered leaflets and bold red banners reading "say no to Occupy Central" affixed to the guardrails.

Nearby, a small group of volunteers from People Power, a pro-democratic political coalition, handed out promotional material. "Hong Kong is so free compared to the mainland – you can't even hand out pamphlets there without getting thrown in a black jail," said Tszho Ng, a 33-year-old volunteer. "So we do what we can to keep the red tide from drowning us. Because if we can't do that with this generation, then it'll be even harder for the next one. That's for sure."

Hong Kong, a former British colony, has been governed under a "one country, two systems" framework since it was returned to mainland control on 1 July 1997. The principle grants Hong Kong's seven million people civil liberties that do not exist on the mainland – rights to free speech and assembly, a free press and an independent judiciary. Yet many Hong Kong people feel that Beijing's influence over the region has grown in recent years, putting these freedoms at risk.

Johnson Yeung, convenor of the Civil Human Rights Front, the group organising the protest, said that he expected half a million people to march on Tuesday – at least as many as in 2003, when the size of the turnout caused Beijing to repeal a controversial proposal for "anti-subversion" legislation.

"My generation, we're pretty pessimistic about our future," said Yeung, a 22-year-old recent graduate. "And we'd like to fight for an alternative." Public sentiment was at its lowest point since 2003, he said.

Over the past year, Hong Kong residents have worried that the region's once free-wheeling press is self-censoring to appease pro-Beijing owners. They fear that communist authorities are meddling in their schools and courts, and that mainland investors have driven up property prices, exacerbating an already enormous wealth gap.

The referendum, Yeung said, had given him hope. Since 1997, Hong Kong's chief executive has been selected by an elite 1,200-person committee handpicked by Beijing. While China's central government has promised Hong Kong a "one person, one vote" system by 2017, it will allow only "patriotic," pro-Communist party candidates to run.

Occupy Central organised the referendum to pressure Beijing into granting Hong Kong a voting process that "abides by international standards for universal suffrage", according to Tai. The ballot – conducted partly online, partly at physical ballot boxes – gave Hong Kong residents three options for how their next leader should be chosen, none of which allowed Beijing to select the candidates.