US President Donald Trump has paid tribute to Charlie Kirk at a memorial service held in an Arizona Stadium, calling the conservative activist a martyr for American Freedom. WATCH THE VIDEO ABOVE: Donald Trump speaks at praises Charlie Kirk memorial service. Know the news with the 7NEWS app: Download todayTaking to the stage Trump said Kirk was “heinously murdered” for his beliefs by a “cold-blooded monster”. “I know I speak for everyone here today when I say that none of us will ever forget Charlie Kirk, and neither now will history.”Trump addressed a key difference between himself and Kirk. A photo of Charlie Kirk is seen on a large screen during his memorial.