CASPER, United States (AFP) - A bitter constitutional row has erupted in a western US town over plans by a church to erect a scathing monument marking the "entrance into hell" of a gay student killed in a vicious hate crime.

The battle erupted after the city council of Casper in the rural state of Wyoming blocked plans by a Kansas pastor to erect the marker in a public park damning the victim of the notorious gay-bashing murder.

Reverend Fred Phelps wants to erect a 1.83-metre (six-foot) granite monument in a park dotted with other markers, including one depicting the Ten Commandments, proclaiming "God condemns gays to hell."

Gay student Matthew Shepard, who hailed from Casper, died in 1998 after mistakenly picking up two men in the town of Laramie who beat him viciously before tying him to a fence overnight on a cold and remote prairie.

The chilling monument was to bear a brass plaque reading: "Matthew Shepard entered hell October 12 1998, at age 21, in defiance of God's warning: 'Thou shalt not lie with mankind as with womankind. It is an abomination': Leviticus 18:22."

Phelps, pastor of Westboro Baptist Church in Topeka in the central state of Kansas, maintains the city is obliged to erect his 15,000-dollar monument, which would bear an image of the young man surrounded by the flames of hell.

He cited a legal precedent obliging cities that display a Ten Commandments monument on public property to also allow other religious statements to be set up in the same area.

But at a stormy meeting Tuesday, the outraged city council unanimously rejected Phelps' proposal, voting instead to move the Ten Commandments monument to a new park where historical documents will be displayed.

"There is no one that wants the Phelps' hate message placed here," said City Manager Tom Forslund, adding that he was "repulsed" by the proposal.

But the decision has failed to quell the row and threatens to open a constitutional and legal battle.

Phelps said he was planning to sue the city for allegedly breaching the First Amendment to the US constitution, which guarantees freedom of speech and religious expression.

"When you attack the First Amendment you have a war in this country!" he said, adding that he would be "first in line" to place his monument in the new park.

The church's lawyer, Shirley Phelps-Roper, was clear: "What they have done is a thinly veiled attempt to do an end run around the constitution," she said.

City officials believe they can win any court battle over the issue.

Phelps has for years campaigned against homosexuality across the conservative Midwest, even protesting with graphic signs at Shepard's funeral, and his antics have provoked anger.

"Matthew was killed by men who believed that hating people because they are different is something to be glorified. And what fueled them is the same form of bigotry and intolerance fueling Phelps," said Elizabeth Birch, of gay civil rights group Human Rights Campaign.

The furor over the monument has engulfed residents of Casper where many want to keep the Ten Commandments monument where it is, but most are put off by Phelps' extreme message.

But another group, Wisconsin-based Freedom from Religion Foundation, has weighed in on Phelps' side, dismissing plans to shift the Ten Commandments to a new plaza as a ploy.

The group threatened its own constitutional challenge to shifting the monument, donated to the city in 1965, and demanded the city remove it from display completely or allow Phelps to erect his anti-gay column.

"It looks like a ruse; it looks like a trick for them to keep it," said the group's Dan Barker.

"It would probably be unconstitutional because the intention of the city is to maintain a religious document."

Phelps meanwhile fumed over the resistance to his marker he faced from Casper city fathers and residents.

The sleepy city of 50,000 residents has been rendered "lawless" as a result of its "sodomite leaders," he said, insisting he would ultimately win the battle.