US President Donald Trump has hailed slain conservative activist Charlie Kirk as a "martyr for American freedom" and vowed at his memorial service to carry on his work, while again accusing what he called the "radical left" for Kirk's murder.

The memorial, organised 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.

Trump said the 31-year-old was targeted because he stood "for freedom and justice, for God, country, for reason and common sense."

"The violence comes largely from the left," the president said without citing any evidence, in a speech that downplayed political violence from the right and often turned starkly partisan.

"He was assassinated

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