Authorities condemned as "invalid and illegal" an unofficial "referendum" on electoral reform in Hong Kong Special Administrative Region (SAR).

Referendum organizers claimed brisk voting, but critics raised suspicions of a possibly inflated turnout due to the flawed online voting system.

The referendum, organized by the Occupy Central movement and run by the Public Opinion Program under the University of Hong Kong (HKU), kicked off its online voting on Friday and will conclude on June 29th.

On Sunday, 15 voting stations opened around the city.

Most voters coming to the voting stations are over 50 and do not know how to use the online system, while some young voters also showed up.

China's top legislature has assured that the SAR's chief executive will be elected through universal suffrage in 2017, with candidates for the post being selected by a nominating committee.

However, the election method has encountered opposition among some Hong Kong residents, who want a public nomination process without candidates being approved by the nominating committee.

The referendum allows residents to choose among three electoral plans, all of which include public nomination, and abstention.

However, Global Times reporters at three polling stations found many elderly residents had difficulty in understanding and differentiating among the proposals offered by pro-democracy groups.

Outside a polling station at Causeway Bay, several pro-democracy groups erected banners, used loudspeakers and handed out booklets to persuade passers-by into joining the referendum and supporting their proposals. On the other side, over a dozen opponents of the referendum from pro-Beijing group Caring Hong Kong Power held banners and shouted slogans, calling for residents to boycott the referendum. The stand-off risked turning into a confrontation before police officers intervened.

As of 10 pm Sunday, organizers said more than 690,000 votes had been cast, among which more than 48,000 were cast at polling stations.

Hong Kong has a population of 7 million.

The turnout was beyond the organizers' expectation of 300,000, but the credibility of the online voting system, which garnered the lion's share of ballots, has been doubted.

Several activists from Caring Hong Kong Power told the Global Times that it was possible to vote online with a fake identity number, therefore the referendum cannot reflect true public opinion.

Local paper Ta Kung Pao also reported that repeated votes could be cast with the same IP address, so it is possible that the pro-democracy groups were exaggerating the total number of ballots.

Tai Yiu-ting, co-organizer of Occupy Central, said that they could not block people who took advantage of the system's loopholes from voting, but it is worth noticing that there were still many people who relished the chance to have a voice

Joseph Cheng Yu-shek, the convener of the Alliance for True Democracy, who promoted the referendum Sunday at the City University of Hong Kong, expected a total of 1 million valid votes in the end, but he admitted that the referendum has "little chance" of being recognized by the Hong Kong government.

"If they [the authorities] do not listen, we will occupy Central, a key financial center in Hong Kong," said Yeung Ching-yin, convener of pro-democracy group Civil Human Rights Front.

Hong Kong Chief Executive Leung Chun-ying said Saturday there was no legal foundation for the referendum.

Yuen Kwok-keung, the SAR's secretary for justice, Sunday said the government respects residents' right to voice their opinions, but must act under the rule of law. He said political reform will not deviate from the Basic Law and the decision of the National People's Congress, reported RTHK.

"Though the public nomination method is a democratic method, it is not in line with the Basic Law and the Hong Kong government will undoubtedly maintain the rule of law and also social stability and it is capable of that," Zhu Shihai, a professor of Hong Kong studies at the Beijing-based Central Institute of Socialism, told the Global Times.

Fung Ka-pun, one convener of Silent Majority of Hong Kong, said most Hong Kong people would not go for the Occupy Central movement, because what most people are concerned about is that every Hongkonger has a vote in the chief executive election and that will be realized in 2017.

Some people expressed the concern that after the vote ends on June 29th, an annual pro-democracy July 1 march in the city will run out of control, despite Tai and the other two organizers emphasizing there would not be any violence.

Yeung revealed that there will be more people protesting on July 1, due to the Occupy Central initiative. The turnout is expected to be more than 100,000 people from over 50 groups.

Police have come up with an emergency plan of mobilizing over 3,000 police officers to deal with any situation, local newspaper Ming Pao reported.