The erosion of press freedom in Hong Kong is a cause for deep concern, the head of Amnesty International said Wednesday, urging residents to guard against a gradual loss of liberties.

Two senior figures from the Hong Kong Morning News Media Group were attacked in March, weeks after Kevin Lau -- a former editor of the liberal Ming Pao newspaper -- was critically wounded in a savage knife assault.

"Amnesty International is deeply concerned about the deteriorating state for freedom of expression in Hong Kong... the erosion of freedom of expression has been gradual and is nonetheless serious for that," the rights group's secretary general Salil Shetty said in a speech.

He said organisations such as Reporters Without Borders and Freedom House noted that China exerts influence over Hong Kong's media -- through direct pressure or indirect interference by editors or owners with interests in mainland China.

"The danger, of course, is that the attacks on freedom of the press could cause people to avoid speaking out and fear will encourage silence," Shetty said, adding there were also grounds for hope.

"One simply has to look at what happened after Kevin Lau was attacked. Something truly remarkable and inspiring happened -- more than 8,000 people marched in the streets of Hong Kong, rallying with the slogans: 'They can't kill us all!'" he said.

"If the goal of the attack was to make Hong Kongers fearful, it failed."

But Shetty said the city should be vigilant against the erosion of freedom of expression.

"Drop a frog into boiling water and it will immediately jump out again, heat the water gently and the frog allows itself to be boiled to death," he said.

"We should not allow the water to be gently brought to boiling point."

The former British colony was guaranteed freedom of the press and freedom of speech, among other rights, under a deal which led to its handover to China in 1997 as a semi-autonomous territory.

But concern is mounting that Beijing is seeking to tighten control over the city and that media freedom is being squeezed as a result.

The New York-based Committee to Protect Journalists said in February media freedom in Hong Kong was currently "at a low point".

It cited self-censorship among reporters, financial and physical threats against the media and legislative steps that could hinder investigative reporting.

Lau was hacked with a cleaver in broad daylight in February by two men who escaped on a stolen motorbike.

The attack came weeks after he had been removed from the top job at liberal newspaper Ming Pao and replaced with an editor widely seen as pro-Beijing.