"Beijing was worried that the amended Public Order Ordinance in the colonial era was not enough to maintain social order. So they wanted to tighten it," Tai said.

The ordinance, which regulates public meetings and processions, was first enacted in 1967 to crack down on the leftist riots against British colonial rule. It outlawed any gatherings of three or more people without police permission.

The ordinance has undergone several changes over the years.

In 1995 it became much more liberal following the enactment of the Bill of Rights Ordinance, based on the United Nations' International Covenant on Civil and Political Rights.

Before then, police permission had to be sought for any public gathering of more than 20 people.

The change meant anyone wanting to hold a protest simply had to notify police. It was then up to officers to raise any objections, according to law scholar Anthony Law Man-wai's book

Then came the eve of the handover … and Beijing's fears of people taking to the streets. The Provisional Legislative Council, which was backed by Beijing, restored the need to get approval for an assembly.

Since 1997, public gatherings of more than 50 people or processions with more than 30 protesters have to tell police seven days in advance and receive a "notice of no objection".

If they don't get the notice they can appeal to a board chaired by a retired judge.

Going ahead without approval leaves organisers open to various charges under the ordinance, including unlawful assembly and unauthorised assembly.

As of September, police had rejected 31 out of 51,946 applications since 1997.

They included a protest against the abuse of police power outside the central government's liaison office in May, which later went ahead after organisers won an appeal.

Critics say police use another tactic to disrupt protests. By withholding approval until the last minute, crowd numbers can be minimised.

Tai said the existing system was a reversal in terms of human rights.

He said even though it was only "barely justifiable" to have to notify police about a protest because it was such a crowded city, protest organisers had proved willing to comply.

Icarus Wong Ho-yin finds himself frequently notifying police of protests as convener of a section of the Civil Human Rights Front that monitors police power.

The front is behind the annual July 1 march that this year drew 400,000, according to organisers, or 63,000, according to the police.

Wong said the application was usually filed in April or May, but the police often did not respond until just days before the event - and then with a list of conditions. For instance, last year there was a ban on musical instruments.

This meant there was a last-minute rush to launch appeals. The musical instruments were allowed in the end, but organisers were ordered to keep amplifiers covered.

"We could be liable if we promote our protests before we receive a police notice of no objection," Wong said.

The ordinance has been challenged, most notably by activist and lawmaker "Long Hair" Leung Kwok-hung, who lodged a judicial review in 2005.

Leung and two others had been convicted of holding an unauthorised assembly in 2002 and put on a three-month good- behaviour bond.

He went to the Court of Final Appeal three years later, challenging the police commissioner's power to restrict the right of peaceful assembly.

The court dismissed the appeal and upheld the convictions.

It ruled the notification system was constitutional and four judges suggested a minor amendment that would make the police chief's power to restrain public assembly constitutional.

Only Mr Justice Kemal Bokhary agreed with Leung that handing the police commissioner the power to restrict the right of peaceful assembly was unconstitutional.

Leung said: "It is better for the police commissioner to seek a court injunction or order if he wants to prohibit a protest."

He said the whole ordinance should be comprehensively reviewed.

A Security Bureau spokesman would not say if a review was likely, saying only that the system had been operating effectively and smoothly.

The Legislative Council in 2000 endorsed 36 to 21 a motion backing the provisions of the ordinance.

But in a report on Hong Kong published in 1999, United Nations Human Rights Committee expressed concern.

"With regard to freedom of assembly, the committee is aware that there are very frequent public demonstrations in HKSAR … the committee is concerned that the Public Order Ordinance could be applied to restrict unduly enjoyment of the rights guaranteed in article 21 of the International Covenant on Civil and Political Rights," it said.

Wong said another area of the ordinance, section 17B on "disorder in public places", had often been used to charge protesters in recent years.

The provision says anyone who acts in a disorderly manner "with intent to provoke a breach of the peace, or whereby a breach of the peace is likely to be caused" could be jailed for 12 months.

Wong said that this was subjective as it was open to police interpretation.

Beijing-friendly lawmaker Ip Kwok-him, chairman of Legco's security panel, said the rise in the number of charges was due to more protests, and that demonstrators were getting more provocative and likely to come into physical contact with police.

"The whole atmosphere of society has changed," he added. "There is much more negative sentiment nowadays."

Asked if the ordinance should be reviewed, he agreed some sections were outdated. But he said it was a sensitive question and he had no further comment.

A police spokesman said the aim of the ordinance was to strike a reasonable balance between the public's right to assemble and express their views and the broader interests of the community at large.

He said the overriding concerns were the preservation of public safety and public order and the protection of the rights and freedoms of others.

"When any one of these pillars is at stake or when there is a breach of the peace, police will discharge the duties prescribed in law with all lawful measures that are considered reasonable in the circumstances," he said.

A Security Bureau spokesman said a recent court judgment had pointed out that it was the government's duty to take reasonable and appropriate measures to enable lawful assemblies to take place peacefully and protect the safety and property of other citizens who might be affected.

Tai said debate over the ordinance should focus on whether the police were respecting human rights in exercising their discretion conferred by the law.

He said another area of the law that could be challenged was the police power to arrange so-called petition areas, often set up far away from the officials protesters wanted to address.

"Are police really respecting or protecting citizens' rights to stage a protest? No, they are failing to do so. They are only achieving the bare minimum standard of human rights," he said.