US President Donald Trump attended a memorial service for prominent American conservative activist Charlie Kirk in Arizona, praising him as a "great American hero" and a "martyr."

Trump was the keynote speaker at the Charlie Kirk memorial service on Sunday, which was attended by senior administration officials, including Vice President J.D. Vance.

"Kirk was killed because he lived bravely. He lived bravely; he argued well," Trump told the crowd in the stadium.

Kirk's wife, Erica, gave a tearful speech and said she had forgiven the man suspected of her husband's murder.

"My husband, Charlie, wanted to save young people, just like the young man who took his own life," she added.