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.