"I moved towards the front of the march with my GoPro and my camera, and what I recall is somebody ran hard into behind me and hit me very hard in the back of my head," he said.
"They were marching, ironically, in front of the Oregon Justice Center -- which houses the sheriff's office and a Portland police precinct -- so we were surrounded by all these institutions that represent the rule of law.
"And yet, what happened was complete anarchy and lawlessness."
The journalist said Portland police did not immediately come to his aid, adding he later watched video of nearly a dozen people pummeling him in the street and dousing him with unknown substances.
"Before I could realize what was happening and remove myself from the situation, the punches and kicks kept coming from every direction," he said.
"When it was happening I didn't know how many people were hitting me. It felt like I couldn't tell if it was five or ten or 20.
"They continued to hit me and they threw what I think were milkshakes and eggs and possibly pepper spray all over me."
Ngo said just before he was attacked, the demonstrators were chanting a phrase he found ironic.
"No hate, no fear," he recalled.
When the police failed to appear, Ngo said he continued to try to break away from the melee.
When the journalist finally reached an area near the Multnomah County courthouse, he said he was told to head back toward where he was attacked.
"In order to get help, I had to walk back in the direction of protesters," he said.
In the aftermath, Portland police released a statement about the protest.
"During today's events, there were multiple assaults reported, as well as projectiles thrown at demonstrators and officers," the statement read.
"There were also reports of pepper spray and bear spray being used by people in the crowd. Officers deployed pepper spray during the incident. There were reports of individuals throwing 'milkshakes' with a substance mixed in that was similar to a quick-drying cement. One subject was arrested for throwing a substance during the incident."
In the wake of the alleged attack on Ngo, one Republican lawmaker has called for an investigation of city officials.
Portland Mayor Ted Wheeler should be investigated by federal law enforcement officials following his handling of the protests, Sen. Ted Cruz, R-Texas, said Sunday.
“To federal law enforcement: investigate & bring legal action against a mayor who has, for political reasons, ordered his police officers to let citizens be attacked by domestic terrorists,” Cruz tweeted alongside a story recapping the incident surrounding Ngo.
Fox News' Greg Norman and Vandana Rambaran contributed to this report.