Erie, Pennsylvania - Barack Obama criticized President Donald Trump's search for an "enemy" to blame for the murder of Charlie Kirk in his first public remarks on the killing that has dominated American discourse over the past week.

The ex-president, who was speaking at an event in Erie, Pennsylvania, called Kirk's death "a tragedy" and said he mourned with the far-right commentator's family.

"The central premise of our democratic system is that we have to be able to disagree, and have sometimes really contentious debates, without resort to violence," Obama insisted.

But he also pointed out the "broader problem" of the language used by Trump and his allies, "who have a history of calling political opponents vermin, enemies who need to be targeted."

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Identifying for enemies to blame, Obama added, would be used as "a rationale for trying to silence discussion around who we are as a country and what direction we should go."

Kirk – whose ideas the 44th president said he considered "wrong" – was fatally shot on September 10 while speaking on campus at Utah Valley University.

Trump, as well as other far-right figures inside and outside his administration, have used the alleged politics of main suspect Tyler Robinson to threaten a widespread crackdown on left-wing groups and dissenting opinions.