Right-wing activist Charlie Kirk has been hailed a “martyr” for his movement at a memorial attended by as many as 100,000 mourners.

US President Donald Trump and Vice President JD Vance are among the speakers due to address the packed football stadium on Monday morning (AEST) in Phoenix, Arizona.

The venue in Kirk’s hometown has a capacity of 63,400 and is jam-packed, with thousands more queuing outside.

The world’s richest man, Elon Musk, was among those watching from the balconies.

“Every seat in this giant arena that isn’t roped off for security is packed to the ceiling. Honoured to be here. All for Charlie Kirk,” he posted to his social media platform X with a video of the crowd.

Trump, who is headlining the service, was expected to be the final speaker.

White House deputy chief of staff Stephen Miller unleashed a furious speech declaring that Kirk was now “immortal”.

“You thought you could kill Charlie Kirk? You have made him immortal,” he shouted.

“They cannot imagine what they have awakened. They cannot conceive of the army that they have arisen in all of us because we stand for what is good, what is virtuous, what is noble.

“To those trying to incite violence against us, those trying to foment hatred against us, what do you have? You have nothing. You are nothing.”

He went on: “You have no idea the dragon you have awakened.

“You have no idea how determined we will be to save this civilisation, to save the West, to save this republic.”

Kirk’s assassination on September 10 at a Utah college campus set off a fierce debate about violence, decency and free speech in an era of deep political division.

The shooting also has stirred fear among some Americans that Trump is trying to harness outrage at the killing as justification to suppress the voices of his critics and political opponents.

Kirk’s death has provoked concerns over Trump and Vance’s calls to punish people who condone or rejoice it. Critics say such speech is protected under the US Constitution’s First Amendment.

Kirk was very close to Trump’s government and his family. The activist founded the Turning Point USA organisation in 2012, with the group now active at schools and universities across the US.

Trump has blamed the “radical left” for Kirk’s death and threatened to go after liberal organisations and donors or others whom he feels are maligning or celebrating Kirk’s death.

Through his platforms – including a successful podcast – Kirk reached an audience of millions, especially young people. Critics accused him of spreading racist, homophobic and sexist views.

Dozens of people, from journalists to teachers, already have lost their jobs as prominent conservative activists and Trump administration officials target comments about Kirk that they deem offensive or celebratory.

The retaliation has in turn ignited a debate over the First Amendment as the administration promises retribution against those who air what are seen as disparaging remarks.

The US ABC network pulled Jimmy Kimmel’s late-night show off the air indefinitely following backlash from affiliated broadcasters and the Trump-appointed head of the Federal Communications Commission over the comedian’s remarks about Kirk.

The US State Department has also warned it will revoke the visas of any foreigners who celebrate Kirk’s assassination.

A 22-year-old Utah man, Tyler Robinson, has been charged with killing Kirk. He faces the death penalty if convicted of the most serious charges.

Authorities have not revealed a clear motive in the shooting, but prosecutors say Robinson wrote in a text to his partner following the shooting that he “had enough” of Kirk’s hatred.

-with AP/AAP