A coalition comprised of more than 40 pro-democracy, religious and human rights groups held the rally to denounce planned anti-subversion laws while others hoped to pressure authorities to issue a white bill detailing the new legislation.
The protesters chanted slogans and waved black balloons and banners carrying the words "Don't want Article 23" as they marched from Victoria Park to deliver a petition at government offices in the business district of Central.
It was the largest protest so far against the controversial legislation. Organisers put the attendance at 25,000 while police said 12,000 joined the march.
"The large number of protesters here shows that many people oppose these laws which we believe are unnecessary at this time," said Jackie Hung of the Justice and Peace Commission, a co-organiser of the demonstration.
The government later issued a statement denying the legislation would curb freedoms.
"The way of life, the rights and freedoms enjoyed by Hong Kong residents, including freedom of speech, will not be affected by legislation to implement Article 23 of the Basic Law," the statement said.
"We note that different views have been expressed over the Government's proposals. We welcome the expression of opinions and are sensitive to people's concerns."
Article 23 of the Basic Law, the mini-constitution of the former British colony since it reverted to Chinese sovereignty in 1997, obliges Hong Kong to pass laws banning treason, sedition, subversion and theft of state secrets.
But there has been an outpouring of concern that the proposed new laws could curtail freedoms in the territory.
"We need to have a democratic system of government in place, with adequate checks and balances before we even consider discussing Article 23," Hung insisted.
Many politicians in the territory are not directly elected but simply handpicked or voted in by small election committees, she noted.
The Basic Law also states that full democracy in the former British colony be introduced by 2007.
Prominent pro-democracy legislator Lee Cheuk-yan said: "We are totally against Article 23 because we believe it is restrictive of freedoms and human rights that we currently enjoy here.
"We do not want to see a security law belonging to China introduced in Hong Kong because state security in China means suppression of the freedom of Chinese citizens so that they cannot even express themselves without fear,"
"We want the government to back down from implementing Article 23 completely, or if not, then at least to not push the law through in such a rash way and in such a hurry. There should be more consultation and a white bill to allow for more discussion."
The government has issued only a vague outline of the laws and many groups have urged the territory to release a white paper providing all the details.
The legislation is due to be enacted after a three-month consultation exercise ends December 24.
Meanwhile Solicitor General Bob Allcock said the laws would probably be reviewed before they are enacted.
Allcock added in a statement broadcast on local radio that concerns expressed from all groups so far had been noted by the government.
Security Secretary Regina Ip said last week it was hoped the analysis of public submissions could be completed by end-January so the bill could be submitted to the Legislative Council, the lawmaking body, in February.
International human rights and press groups last Tuesday condemned the planned legislation, charging it would jeopardise fundamental freedoms in the territory.
Last week, a survey conducted by the Hong Kong Transition Project, an academic-led group which monitors the effects of the handover from British to Chinese rule in the territory, found the proposed security laws had raised fears about personal freedoms to their highest levels since 1997.
Some 10 percent of respondents said they were "very worried" about personal freedom compared to five percent three months earlier, while 70 percent called for the publication of a white bill.
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