An employee at a high-end Chicago cocktail bar was questioned by the Secret Service on Tuesday evening after allegedly spitting on Eric Trump, according to local media reports and an interview with another worker at the establishment.
The president's son confirmed the incident to Breitbart News.
"It was purely a disgusting act by somebody who clearly has emotional problems," Trump, 35, told the conservative site. He declined to comment to The Washington Post.
Trump, who is executive vice president of the Trump Organisation, was at the Aviary around 8.30pm on Tuesday when he was allegedly spat on by a waitress, NBC reported.
The Secret Service declined to comment on the incident. Anthony Guglielmi, a Chicago Police Department spokesman, confirmed on Twitter that local officers assisted the Secret Service at the bar but declined to discuss what happened.
Aviary, in Chicago's West Loop, said on Wednesday that "unfortunate incident" involving Trump and one of its employees occurred and said the unnamed employee was placed on leave.
"What is certain is this: no customer should ever be spit upon," the restaurant said in a statement obtained by local NBC reporter Mary Ann Ahern.
The restaurant has been bombarded since the incident, it said, with "hundreds" of people calling to threaten employees and post fake reviews online. Aviary also said it was troublesome that others praised the incident as an act of civil disobedience.
"A degrading act lowers the tenor of debate," Aviary said.
Yelp appeared to remove some negative reviews that flooded the restaurant's page. It later posted a warning to users that recent news reports caused an uptick of people visiting to weigh in.
Restaurants and bars have regularly become hostile territory for Trump family, staffers and allies in recent years, as the establishments' owners workers and customers have harassed or chased out everyone from Sen. Ted Cruz, R-Texas, to Sarah Sanders, sparking debate about what constitutes appropriate political protest, and whether such battles should extend to the dinner table.
Eric Trump attributed the alleged spitting incident to the employee's politics.
"For a party that preaches tolerance, this once again demonstrates they have very little civility," Trump told Breitbart, apparently referencing Democrats. "When somebody is sick enough to resort to spitting on someone, it just emphasises a sickness and desperation and the fact that we're winning."
Aviary has already waded into the national political discourse. The boundary-pushing establishment that has nabbed a James Beard Award for its bar program. Its co-owner, Nick Kokonas, made headlines in January by offering the Clemson Tigers, college football's national champions, "an actual celebration dinner" at his restaurant Alinea after President Donald Trump served the players a banquet of fast food at the White House.
After the bar closed for the night, another worker at the Aviary confirmed the basic details of the incident to The Post, though they declined to speak on the record.
Hours after the incident, Eric Trump took to Twitter to post photos of his family's hotel property along the Chicago River, making no reference to what happened at the Aviary.