The 22-year-old student, Matthew Shepard, died of injuries he received when he was beaten, burned and tied to a fence for 18 hours on Wednesday night.

Matthew Shepard: Died of his injuries

"In the face of this terrible act of violence, they are joining together to demonstrate that an act of evil like this is not what our country is all about," Mr Clinton said in a statement released by the White House.

He said existing hate crimes legislation, which covers attacks motivated by religious or ethnic hatred, should be extended to cover sexual orientation, gender and disability.

Gay-rights activists said the attack shows the need for tougher hate-crime legislation, and reveals a growing national level of intolerance.

"There is a climate right now of intolerance that we believe is being fostered by religious political organisations," said Kim I Mills of the Human Rights Campaign, the largest US gay and lesbian political group.

Charges of attempted murder

Two young men have been charged with attempted first-degree murder, kidnapping and aggravated robbery. Two women have been charged with being accessories after the fact.

Russell Henderson and Aaron McKinney: Charged with attempted murder

Police said the two men lured Mr Shepard from a University of Wyoming campus bar by telling him they were gay. The three allegedly drove off in a truck, where Mr Shepard was beaten. Later, the assailants tied him to the fence and beat him some more.

Authorities said the two men made anti-gay remarks to the two women.

Police said there were indications the attack was both a robbery and a hate crime, and the investigation was continuing.

Condition deteriorating

The hospital officials said Mr Shepard had suffered severe head injuries in the attack, including damage to his brain stem.

"When he arrived on Wednesday night ... his head trauma consisted of a massive blow to the right side of his head," said Rulon Stacey, representative of the hospital.

"It fractured his skull from behind his head to in front of his right ear and compressed his skull into his brain," he said.

Mr Shepard's parents flew in to be with their son, from Saudi Arabia where his father works in the oil industry.

The parents said in a statement that their son's one intolerance was with people who "don't accept others as they are".

"He has always strongly felt that all people are the same regardless of their sexual preference, race or religion," they said.