US President Donald Trump has called right-wing activist Charlie Kirk a "martyr for American freedom," at a memorial service in Arizona, saying he was killed by a "radicalised, cold-blooded monster for speaking the truth."

Tens of thousands of people attended the memorial at State Farm Stadium in Glendale, where Kirk last lived with his family.

Trump said the 31-year-old was targeted because he stood "for freedom and justice, for God, country, for reason and common sense," adding that the attack had made Kirk "immortal."

"He was assassinated because he lived bravely, he lived boldly and he argued brilliantly," he said.

"He's a martyr now for America's freedom. I know I speak for everyone here today when I say that none of us will ever forget Charlie. And neither now will history."

Kir

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