Hong Kong journalists vowed Thursday not to be intimidated by a savage attack on a veteran colleague that has stoked fresh concerns for media freedom, declaring "they can't kill us all".

Kevin Lau -- former editor of the liberal newspaper Ming Pao -- is in critical condition in hospital after two men attacked him with a cleaver, sparking condemnation from the United States, the European Union and press groups.

The attack came amid mounting concern that China -- which also condemned the attack and offered words of support to Lau and his family -- is trying to tighten its control over the semi-autonomous territory.

Lau was removed as editor at the daily last month, triggering protests by staff. They feared that replacing him with an editor from Malaysia seen as pro-Beijing was an attempt to stifle the paper's strong track record of investigative reporting.

On Sunday protesters staged a demonstration in support of press freedom.

Journalists took to social media Thursday to express support for Lau, saying they would not be deterred from doing their jobs.

"They can't kill us all," read a widely-shared banner on Facebook, accompanied by a graphic featuring three fists clutching a pencil, a smartphone and a microphone, representing a journalist's tools.

"We are angry. We roar. We need to stand up," said a statement by a group of university journalism students on the social network.

At the Chinese University, where Lau taught journalism part-time, banners and flyers featuring the slogan were displayed.

Chan Yuen-man, a journalism lecturer there, told AFP that freedom of the press cannot succumb to "pressure or the invisible hand".

Ming Pao's usual red logo was coloured black on Thursday.

"My colleagues won't be scared because of this incident, we will continue with our work," wrote the newspaper's interim chief editor Cheung Kin-por.

Around 200 people held a candlelight vigil while wearing black to mark what they called the loss of press freedom. They wrote slogans such as "Condolences to press freedom" on the ground with white chalk at government headquarters late Thursday.

"It is incredibly absurd that freedom of the press can be supressed this way. People should stand up immediately," 19-year-old university student Leung Kai-ping, a participant at the protest, told AFP.

Lau, who was known for his uncompromising political investigations, was slashed six times with a cleaver.

Security cameras showed the suspects riding a motorcycle. No arrests have been made so far.

Police described the attack as "a classic triad hit, which was designed to maim, not kill, to send a warning", the South China Morning Post reported.

Lau has undergone surgery for wounds including a 16 centimetre-long (six-inch) gash that cut through his back muscles and remained in critical condition, a Hong Kong government spokesman said.

"Kevin is conscious...and the doctors have said that he doesn't need any more surgery at the moment," a statement from his family said.

Hong Kong Chief Executive Leung Chun-ying told reporters Lau had made progress.

"We are very concerned about the incident involving Mr Lau and strongly condemn the actions of the assailant," the deputy director of Hong Kong's Chinese liaison office Yang Jian told reporters late Thursday.

The US consulate has said it was "deeply concerned" as it joined calls from media groups for authorities to bring the perpetrators to justice.

On Thursday the European Union's office in Hong Kong said it was "shocked by the cruel attack" and welcomed Leung's insistence that the territory would not tolerate such violence.

The International Consortium of Investigative Journalists, which was behind a recent expose on the offshore accounts of China's elites on which it worked with Ming Pao, said it was "horrified".

The group said it had no evidence linking the attack to the probe into powerful mainland figures, but speculation over a connection "does reflect the real concern and anxiety felt by many in the Hong Kong press corps".

In June last year there were several attacks against employees of the outspoken Apple Daily. Chen Ping -- a publisher of a magazine known for its outspoken coverage of mainland issues -- was also beaten up.

Earlier this month, the New York-based Committee to Protect Journalists said media freedom in Hong Kong was currently "at a low point", while Paris-based Reporters Without Borders said its independence "is now in jeopardy".