US President Donald Trump hailed slain conservative activist Charlie Kirk as a "martyr for American freedom" on Sunday and vowed at his memorial service to carry on the activist's work while again suggesting the "radical left" was responsible for Kirk's murder.

"The violence comes largely from the left," Trump said, without citing any evidence, in remarks that often turned starkly partisan in contrast to the more solemn tone that most other speakers adopted.

Trump's messaging reflected the dual nature of the event, which had the feel of a religious revival mixed with a "Make America Great Again" rally.

The memorial, organised 11 days after he was assassinated by Kirk's conservative youth advocacy organisation Turning Point USA, drew tens of thousands of mourners dressed in red, white and blue who filled State Farm Stadium in Glendale, Arizona.

Kirk's friends and fellow conservatives praised him as an inspirational Christian leader who founded a political movement they vowed to nurture.

His wife, Erika, who has taken the helm of Turning Point, delivered an emotional tribute to her late husband, looking up at the heavens and mouthing, "I love you," before speaking about his devotion to Christianity, his family and his activism.

The Kirks have two young children.

"I want all of you to know, while Charlie died far too early, he was also ready to die," she said. "He left this world without regrets. He did 100% of what he could every day."

She also offered forgiveness to the 22-year-old man who has been charged with Kirk's murder, citing the Bible's account that Jesus Christ urged his followers to forgive his tormentors while on the cross.

"My husband Charlie wanted to save young men, just like the one who took his life," she said, before adding tearfully, "I forgive him" as the crowd applauded solemnly.

Some political figures cast Kirk's death as a pivotal moment in the conservative movement, exhorting followers to finish the work he began in sometimes aggressive language.

"We will carry Charlie and Erika in our heart every single day, and fight that much harder because of what you did to us," Stephen Miller, the powerful White House adviser, said in a fiery speech.

"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 the republic."

The memorial featured a number of leading Christian rock artists, giving it the air at times of a megachurch Sunday service.

As music filled the arena, some men and women closed their eyes and swayed with their arms in the air, tears rolling down their cheeks.

The arena, which normally has a capacity of 63,000, appeared completely full.

Other speakers included Secretary of State Marco Rubio and Secretary of Defense Pete Hegseth.

Vice President JD Vance credited Kirk with helping get Trump elected last year by mobilising young voters.

"Our whole administration is here, but not just because we love Charlie as a friend, even though we did, but because we know we wouldn’t be here without him,” Vance said.

Civil rights groups have criticised Kirk for rhetoric they described as racist, anti-immigrant, transphobic and misogynistic, while his backers say he was a defender of conservative values and a champion of free speech.

Kirk, 31, was killed with a single bullet as he answered an audience member's question at a campus event in Utah organised by Turning Point.

A 22-year-old technical college student has been charged with Kirk's murder, and investigators say he told his romantic partner in text messages that he had killed Kirk because he had "enough of his hate." (Reuters)