Hong Kong journalists said Sunday the past year had been the "darkest" for press freedom for decades, with growing political tensions expected to worsen the situation in the future.

Concerns over freedoms in the semi-autonomous Chinese city have grown this year following several attacks on journalists -- including February's brutal stabbing in broad daylight of a former editor of a liberal newspaper, Kevin Lau.

"The year under review has been the darkest for press freedom for several decades," the Hong Kong Journalists Association (HKJA) said in a statement accompanying its annual report.

Reporters have faced attacks "both physical and otherwise" as the political environment becomes increasingly heated, with the city deciding on how to choose its next leader in 2017, it said.

Residents will be able to vote for the chief executive in 2017, but China has rejected the idea of giving them a say in who can stand for the post. Many pro-democrats fear Beijing will hand-pick sympathetic candidates.

The report, entitled "Press freedom under siege", said advertising boycotts were launched against liberal news outlets such as the Apple Daily newspaper.

The statement also said Beijing has been "lecturing" media outlets on the need to increase coverage to oppose certain democratic movements and to report on economic developments in the mainland.

"As political tension between Hong Kong and Beijing increases, the HKJA expects further deterioration in press freedom in the years to come," it said.

"Previously when they deal with you it was like boiling a frog with warm water, but no longer. Now they will cook the frog directly in hot water: when they do this they can stomp on press freedoms without restraint," HKJA chairperson Sham Yee-lan told a press conference.

Last Tuesday tens of thousands took to the streets for a pro-democracy march, and more than 500 people who staged an overnight sit-in afterwards were arrested.

Five organisers of the march were also arrested for obstructing police.

Organisers said more than half a million people marched, while monitors put the figure between 140,000 and around 170,000.

Anger in the city has grown following a white paper published by China last month that reaffirms its control over Hong Kong.

The city was handed back to China by Britain on July 1, 1997 under a "one country, two systems" agreement, which allows residents civil liberties not seen on the mainland, including free speech and the right to protest.