Eight people who US prosecutors say have ties to Antifa have collectively been sentenced to 450 years in prison for their roles in a riot outside an immigration detention centre in Texas.

A former US Marine Corps reservist, who was convicted of attempted murder of a law enforcement officer, was sentenced to 100 years in prison. The others' sentences range between 30 to 70 years, according to the justice department.

Last year, President Donald Trump designated Antifa - short for anti-fascist - a "domestic terrorist organisation" .

A US judge called their actions on 4 July "an assault on democracy", while their families condemned the length of their sentences.

"The sentences handed down today make clear that Antifa terrorists who attack law enforcement and federal facilities will face swift and uncompromising justice," Acting US Attorney General Todd Blanche said in a statement, condemning violent extremism.

The US Department of Justice (DOJ) said the North Texas Antifa Cell operatives began shooting off and throwing fireworks at the facility in Prairieland, just south of Dallas, on the Independence Day holiday last summer, as well as vandalising vehicles and a guard kiosk on the property.

The eight were convicted on an array of charges, including rioting, using weapons and explosives, providing material support to terrorists, and obstruction.

Benjamin Hanil Song, the purported leader of the group, was sentenced to 100 years in prison. According to the Justice Department, he had faced a minimum penalty of 20 years.

Song said in a written statement on Tuesday that he fired his gun because he believed the police officer was preparing to shoot a protester, according to multiple media outlets and an advocacy group for the defendants.

His mother, Hope Song, has disputed claims that he shot the officer and said he didn't intend to hurt anyone, according to the Associated Press.

Maricela Rueda was sentenced to 70 years in prison; Cameron Arnold (also known as Autumn Hill), Savanna Batten, Zachary Evetts, Bradford Morris (also known as Meagan Morris) and Elizabeth Soto were each sentenced to 50 years in prison; and Daniel Rolando Sanchez-Estrada was sentenced to 30 years in prison.

A ninth defendant, Ines Soto, will be sentenced on 1 July. Seven others involved in the incident pleaded guilty before the trial to one count of providing material support to terrorists. They will also be sentenced on 1 July.

In a statement on Tuesday, the DOJ said the North Texas Antifa Cell was part of a network of individuals and small groups ascribing to an ideology that calls for the overthrow of the US government, law enforcement and the system of law.

The defendants' supporters say the punishments were unduly harsh.

While other immigration facilities have seen numerous protests, as well as visits from Democratic lawmakers, the Prairieland site had been relatively free of disruptions.

Some of the protest's attendees said they believed that night was supposed to be a "noise demonstration" that did not include violence.

The defendants have denied any affiliation with Antifa and have said they attended the demonstration to support detained immigrants.

Antifa has long been a target for Trump. In September, he signed an executive order designating the loosely organised, far-left movement as a domestic terror organisation.

Critics of the designation have said that Antifa is an ideology, not an actual group with an organised leadership structure or membership, and that subscribing to a specific ideology is not a crime.