Caustic editorials, cyber-attacks and police displays form part of mainland's response to vote on right of HK to choose own leader

About 730,000 Hong Kong residents – equivalent to a fifth of the registered electorate – have voted in an unofficial "referendum" that has infuriated Beijing and prompting a flurry of vitriolic editorials, preparatory police exercises and cyber-attacks.

Occupy Central with Love and Peace (OCLP), the pro-democracy movement that organised the poll, hopes to pressure Beijing into allowing Hong Kong's 7.2 million residents to choose their own leader by 2017. If Beijing refuses, OCLP says, the movement will mobilise at least 10,000 people next month to block the main roads in Central, a forest of skyscrapers housing businesses and government offices on Hong Kong island's northern shore.

Mainland officials and newspapers have called the poll "illegal" while many have condemned the OCLP, claiming it is motivated by foreign "anti-China forces" and will damage Hong Kong's standing as a financial capital. The movement is led by Benny Tai Yiu-ting, an assistant law professor at the University of Hong Kong.

Around 3.5 million of Hong Kong's residents were registered to vote in its last official election in 2012, when polls were held for the legislative assembly, with 1.8m ballots cast in total.

On Tuesday, Zhang Junsheng, a former deputy director of Xinhua News Agency in Hong Kong, called the poll "meaningless". The state-run Global Times newspaper – notorious for its nationalistic editorials – mocked the referendum as an "illegal farce" and "a joke". The territory's pro-Beijing chief executive, Leung Chun-Ying, said: "Nobody should place Hong Kong people in confrontation with mainland Chinese citizens."

Mainland censors have meanwhile scrubbed social media sites clean of references to the OCLP.

Since Britain handed Hong Kong back to mainland China in 1997, Beijing has governed the region under a principle of "one country, two systems". The framework allows Hong Kong to maintain an independent judiciary and press, even as Beijing retains control over many of its other administrative functions, including foreign affairs and defence.

Yet many Hong Kong residents feel that their independence is eroding as Beijing's influence grows. Since the 1997 handover, Hong Kong's chief executive – the territory's highest-ranking official – has been selected by a committee of 1,200 local elites, many of them hand-picked by Beijing. While authorities have promised to grant Hong Kong universal suffrage in 2017, they will only allow "patriotic" candidates to run in the election, rendering the voting process moot.

The OCLP poll, which went live on June 20 and will end on Sunday, gave Hong Kong residents three options on how they could choose their next chief executive. All three would allow the public to nominate its own candidates.

Beijing has shown no signs of backing down. Earlier this month, a powerful government office issued a white paper claiming "comprehensive jurisdiction" over the territory. "The high degree of autonomy of the HKSAR [Hong Kong Special Administrative Region] is not full autonomy, nor a decentralised power," it said. "It is the power to run local affairs as authorised by the central leadership."

Michael DeGolyer, director of the transition project at Hong Kong Baptist University, one of the territory's most respected polling organisations, said: "It's very clear from surveys that the vast majority of the people voting in this referendum are doing it as a reaction to this white paper – particularly because they see it as threatening the rule of law ... That's not negotiating on the one country two systems principle, that's demolishing it."

He added that many Hong Kong residents see the OCLP as a radical, and potentially destabilising force. "They're not voting by the hundreds of thousands that they're going to attack police or stand in the middle of the road," he added. "That's not going to happen."

As the poll opened, it was quickly hit by what one US-based cyber-security firm called the "most sophisticated onslaught ever seen".

"[The attackers] continue to use different strategies over time," Matthew Prince, the chief executive of CloudFlare, a firm that helped defend against the attack, told the South China Morning Post. "It is pretty unique and sophisticated." The firm could not identify the origin of the attack.

On Wednesday, officers at Hong Kong's Police College took part in a "major exercise" to prepare for OCLP planned protest. "Swift action will be taken if protest groups try to paralyse traffic, occupy infrastructure facilities and block emergency access," an unidentified source told the newspaper.