US police hail citizen who overheard and reported a conversation indicating an attack was being planned on an Independence Day celebration in southeastern Richmond city, leading to two arrests and seizure of multiple guns.

"One phone call saved numerous lives on the Fourth of July," Richmond officials say. (AP)

Authorities thwarted a planned July 4 mass shooting after receiving a tip that led to the arrest of two men and the seizure of multiple guns, police in Virginia state's Richmond city said, an announcement that came just two days after a deadly mass shooting on the holiday in America's Illinois state.

A "hero citizen" overheard a conversation indicating there was an attack being planned on an Independence Day celebration in the capital city and called police to report it, police chief Gerald Smith said at a news conference on Wednesday.

The caller said the attack was planned for the Dogwood Dell Amphitheater, where an annual fireworks show is held, Smith said.

"One phone call saved numerous lives on the Fourth of July," Smith said.

Police initiated an investigation along with the US Department of Homeland Security and the FBI, which led to the arrests of two men on charges of being non-US citizens in possession of a firearm, Smith said. He said additional charges are possible.

Julio Alvarado-Dubon, 52, was arrested on July 1, the same day police received the tip, Smith said. 

He said police put a second suspect, Rolman Alberto Balacarcel, 38, under surveillance beginning that day, but did not initially have probable cause to arrest him. He was arrested on Tuesday in Albemarle County, near Charlottesville, and was being held in a local jail.

Police spokesperson Tracy Walker said both men are from Guatemala. 

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'Keep your head on a swivel'

Richmond Mayor Levar Stoney, who joined the news conference along with members of the city council, decried what he called an epidemic of gun violence across the country.

"Whether you’re at home in your cul-de-sac, or in your neighbourhood, or in a park, or at a parade, out dining — you have to keep your head on a swivel," Stoney said. "And that’s not the country that I know I desire to live in ... but those are the facts of the matter at this moment."

Smith said the apparent plot was unconnected to another shooting that wounded six people in Richmond in the early morning hours of July 4.

The same day, a gunman opened fire from a rooftop during a Fourth of July parade in Highland Park city, killing seven people and wounding more than three dozen.

Robert E Crimo III was charged with seven counts of murder on Tuesday. The shooting sent hundreds of people fleeing in fear and set off an hours-long manhunt. Authorities have not yet identified a motive in that shooting.

READ MORE: US police arrest suspect in July 4 mass shooting

Source: AP