Donald Trump Jr. was heckled — by his father’s own supporters — during a book-tour stop over the weekend, a report said Monday.

President Trump’s eldest son was on stage in a lecture hall at the University of California on Sunday to promote his new tome, “Triggered: How The Left Thrives on Hate and Wants to Silence Us,’’ when the 450 people in the audience were told there would be no Q&A session with him “due to time constraints,’’ the Guardian said.

Organizers for the event told The Post on Monday that they had already decided days beforehand not to do a Q&A because “we were made aware of a pre-planned effort to disrupt” the gathering.

The audience included some of the White House’s most conservative supporters, who had chanted “USA! USA!” when Trump Jr. walked on stage, only to soon start yelling, “Q&A! Q&A!”

They refused to stop, even as Trump Jr. tried to argue that if he did a question-and-answer session, the Republican administration’s liberal enemies would twist his answers to suit their own agendas.

But the discontented group — apparently members of the fringe America Firsters, who think the president is too soft on such issues as trade and immigration — drowned him out, the Guardian said.

After about 20 minutes of futile back-and-forth, Trump Jr.’s frustrated girlfriend, Kimberly Guilfoyle, who was with him on stage, jumped in and chided the audience, the site said.

“You’re not making your parents proud by being rude and disruptive!” she shouted at the college crowd. “I bet you engage and go on online dating because you’re impressing no one here to get a date in person.”

She and Trump Jr. left the stage moments later, the site said.

“We wanted to ask questions about immigration and about Christianity, but they didn’t want to face those questions,” said a man, who claimed the president is being co-opted by “fake conservatism,” of Trump Jr.

Event organizers said Trump Jr. was on stage for 35 minutes, which was the amount planned.

“The event was not cut short or ended early, as in lieu of the Q&A we simply extended the amount of time given to our VIPs for their remarks,” said a rep for the organizers, Turning Point USA.

The organization says on its Web site that it is a non-profit designed to “identify, educate, train, and organize students to promote the principles of freedom, free markets, and limited government.”