Hong Kong journalists have condemned an attack on two cameramen outside the Beijing home of the wife of jailed Nobel laureate Liu Xiaobo, slamming it as a violation of press freedom.

The pair were beaten up by a group of unidentified men when they were filming an activist's attempt to visit Liu Xia, who is herself under house arrest, at her apartment building on Friday.

One of the cameramen was punched in the face and pushed to the ground, while the attackers also tried to snatch the camera from the other journalist and hit him in the head, Hong Kong TV news footage and reports said.

"The violence is a serious infringement of press freedom," the Hong Kong Journalists Association said in a statement late Friday.

While attacks on journalists are not new on the mainland, the association said the degree of violence in the latest assault showed "the situation is getting worse".

Hong Kong activist Yang Kuang who was trying to visit Liu Xia was apparently taken away in a police car hours later and his whereabouts remain unknown, the South China Morning Post reported Saturday.

The security guards at Liu Xia's apartment had refused to let Yang enter, before a group of men came out to push away him and yelled abuse at the journalists and set upon the two cameramen, the Post said.

A spokesman for the Hong Kong government said it was "highly concerned" over the incident, and that the right to report on the mainland must be respected.

Liu Xia has been held under house arrest since her husband -- who was sentenced to 11 years in jail in 2009 for "subversion" after co-authoring a bold petition calling for reforms -- won the peace prize in October 2010.

In December, Chinese activists including top dissident Hu Jia broke through a security cordon to visit Liu Xia in a daring affront to the authorities, the first time in more than two years that friends have been able to visit her.