Alabama Supreme Court Justice Roy Moore said he had no intention of removing a two-tonne granite monument of the Ten Commandments from a public building.
But a federal court ruling by US District Judge Myron Thompson says it violates a constitutional prohibition on government promotion of religion.
The federal court ordered the removal of the display placed in the state justice building in Montgomery by 20 August. But Moore has asked the US Supreme Court to squash Thompson's order.
"Civil disobedience is the right of all of us when man's law causes us to break God's law," controversial minister Jerry Falwell told the crowd, comparing Moore's actions to those of civil rights leader Martin Luther King Jr.
Moore, a Christian who was elected chief justice of this Bible Belt state three years ago, has won support in fundamentalist circles for championing public displays of the Commandments and challenging those who oppose such efforts.
"We feel our God has been insulted," Rick Scarborough, a former pastor from Texas, said. "Judicial tyranny has drawn a line in the sand. We will not go any further in expurgating God from our culture."
Moore first came to national attention in the late 1990s when he waged a legal fight to keep a small hand-carved plaque of the Commandments on display in his courtroom in Gadsden, Alabama.
After his election to the state's highest court in 2000, he had the controversial 2,268 kg domed granite monument moved into the judicial building without any public announcement.
It contains two tablets bearing the commandments that, according to Judeo-Christian scripture, were passed from God to Moses.
Quotes from American founding fathers Thomas Jefferson and George Washington and other historical figures are inscribed on its sides.
Civil liberties groups have objected to the display and fought a legal battle to have it removed.
"Judge Roy Moore is setting a very bad example," said Larry Darby, a spokesman for American Atheists. "He should remove this rock from taxpayer property and resign as chief justice."