Posts on social media by supposed antifa leaders urged members to meet at the Pennsylvania site on 4 July.

In response, far-right groups assembled on the historic grounds on Saturday - but no adversaries showed up.

The holiday marks the US declaration of independence from Britain in 1776.

The posts that appeared on social media ahead of the Independence Day celebrations reportedly called for people to flock to the site in face paint.

Activists would "be giving away free small flags to children to safely throw into the fire", the hoax call suggested.

"Let's get together and burn flags in protest of thugs and animals in blue," one anonymous Facebook post said.

There is no evidence to suggest that those responsible for the online posts are in any way linked to antifa - a loosely-affiliated network of mainly far-left activists.

Hours before the flag-burning was supposed to start, far-right groups gathered in a parking lot next to a Wal-Mart. Some were armed.

"It doesn't matter if it's a hoax or not," Christopher Blakeman, who travelled to the Gettysburg site from West Virginia on Saturday, told the Washington Post newspaper.

"They made a threat, and if we don't make our voices heard, it'll make it seem like it's OK," he added.

In his Independence Day address on Saturday, President Donald Trump vowed to defeat what he called the "radical left" as protests against racism and police brutality continue.

Mr Trump said he would "fight... to preserve the American way of life", while railing at "mobs" targeting historical monuments.

In May, the president said the US would designate antifa, which he accused of starting riots at street protests over the death of African American George Floyd, a terrorist organisation.

Fourth of July: Trump vows to defeat 'radical left' in Independence Day speech

US President Donald Trump has used an Independence Day address to vow to defeat the "radical left" as protests sweep the country.

Striking a combative tone, Mr Trump said he would "fight... to preserve American way of life", while railing at "mobs" targeting historical monuments.

Ahead of his speech, Black Lives Matter protesters gathered nearby.

Mr Trump's 2020 election rival Joe Biden said everyone deserved "a full share of the American dream".

The 4 July holiday marks the nation's declaration of independence from Britain in 1776 and is one of the most important days in the national calendar.

Historically presidents have used the occasion to deliver speeches extolling the virtues of unity. Last year Mr Trump spoke of the "extraordinary heritage" of the country at an event with a militaristic theme that involved Air Force flyovers and tanks parked on display.

This year, Mr Trump's address was again followed by a flyover involving various aircraft, including B-52 bombers and F-35 fighter jets.

Speaking from the White House lawn on Saturday, Mr Trump took aim at protesters that he sees as anti-patriotic who have taken to the streets in the wake of the death in police custody in May of African-American George Floyd.

"We are now in the process of defeating the radical left, the anarchists, the agitators, the looters, and the people who, in many instances, have absolutely no clue what they are doing," he said.

"We will never allow an angry mob to tear down our statues, erase our history, indoctrinate our children."

Statues and monuments of historical figures associated with racism or slavery have been pulled down or removed amid the wave of protests - drawing Mr Trump's ire.

"Their goal is demolition," he declared.

Elaborating on his plan to create a "National Garden of American Heroes" featuring statues of renowned Americans, Mr Trump said the country's rich heritage belonged to citizens of all races.

"The patriots who built our country were not villains," he said. "They were heroes."