Virginia's governor has urged people to stay away from a planned weekend rally of far-right and white supremacist groups in the university town of Charlottesville.

Charlottesville, an otherwise sleepy city of 46,000 in central Virginia, is bracing for an influx of hundreds of far-right activists - among them white supremacists and neo-Nazis from across the United States - on Saturday to protest against the planned removal of a Confederate monument.

The National Guard has been put on alert because of the risk of violence during the "Unite the Right" rally.

"In advance of [Saturday's] rally there have been communications from extremist groups, many of which are located outside of Virginia, who may seek to commit acts of violence against rally participants or law enforcement officials," Governor Terry McAuliffe said in a statement.

"In the event that such violent or unlawful conduct occurs, I have instructed state public safety officials to act quickly and decisively in order to keep the public and themselves safe.

"Many of the individuals coming to Charlottesville are doing so in order to express viewpoints many people, including me, find abhorrent.

"I want to urge my fellow Virginians who may consider joining either in support or opposition to the planned rally to make alternative plans."

Thousands of white nationalists, including supporters of the Ku Klux Klan (KKK) white supremacist group, and anti-fascist activists are expected to turn out in Charlottesville, the town planning to remove a statue of General Robert E Lee, who led Confederate forces during the US Civil War.

"The Charlottesville event could be a potentially historic showcase of hate, bringing together more extremists in one place than we have seen in at least a decade," said Oren Segal, director of the Center on Extremism at the Anti-Defamation League, a group that monitors and combats anti-Semitism.

Governor McAuliffe, a Democrat, said he had given security forces instructions to act quickly and decisively if violence breaks out.

READ MORE: Alt-right rally - Charlottesville braces for violence

Units of the Virginia National Guard will be placed on standby, he added.

Before the rally, David Duke, the former Imperial Wizard of the KKK, called on his supporters to descend on Charlottesville.

The League of the South also urged its members to be prepared for violent confrontations with anti-fascists and the Black Lives Matter civil rights movement.

In a blog post on its website, the neo-Confederate organisation said the rally would "affirm the right of southerners and white people to organise for their own interests just like any other group is able to do, free of persecution".

In recent months, Confederate monuments like the statue in Charlottesville have become key venues for the alt-right and other far-rightists.

In May, alt-right leader Richard Spencer led a rally of torch-wielding far-right activists at the monument, although several hundred counterprotesters outnumbered them.

On July 8, a few dozen Ku Klux Klan marchers gathered in Charlottesville to protest plans to remove the statue of Lee. But they were outnumbered by hundreds of jeering counterprotestors.