Serbian police say the man, who has been identified only as D.H., was beaten up by unknown assailants in downtown Belgrade early on Saturday morning.

Jovanka Todorovic, from the organization Labris which hosted the conference on LGBT (lesbian, gay, bisexual and transsexual) rights, told Associated Press the man was attacked by a group of young men who bashed him with a glass ashtray.

He was taken to Belgrade's emergency hospital suffering life-threatening head injuries and internal bleeding.

"He has undergone surgery and been put in intensive care as his condition is very serious," Dusan Jovanovic, the hospital's deputy director, told news agency AFP.

Serbia's Interior Minister Nebojsa Stefanovic said in a statement that he had ordered an "intensive investigation" to bring the perpetrators to justice.

"We will not allow these kinds of things to remain unpunished ... and we will arrest the German citizen's attackers," he said.

The incident is not unique in Serbia. The country has a long history of violence against gay rights campaigners by far-right groups - a serious challenge for the government, which is currently seeking EU membership and has pledged to improve its protection of human rights.

Later on Saturday, anti-riot police were on hand as hundreds of gay rights activists marched through central Belgrade in protest of the latest attack.

The gay pride parade is scheduled to take place in two weeks and will be the first event of its kind since a gay pride march in 2010 resulted in clashes between police and ultra-nationalists that left more than 100 people injured.

After clashes in 2010, authorities banned such marches, citing security concerns that participants may be attacked by right-wing extremists.

nm/sb (AFP, AP)