Far-right demonstrators and "antifa" protesters clashed in OR over the weekend, resulting in multiple injuries and arrests, authorities said.
"I can't just sit by and watch my friend be brutally attacked", he added. The tweet was posted hours before Ngo took to Twitter to reveal he was violently attacked.
Mr Ngo is an editor at online magazine Quilette but opponents characterise him as a far-right campaigner and criticised him over a Wall Street Journal article published a year ago entitled: "A visit to Islamic England".
Dan O'Sullivan, who has written for Rolling Stone and VICE, among other mainstream outlets, said in a tweet, "Andy Ngo is not a journalist, and in any event he went there hoping for exactly that outcome and for tweets like this". At least three people were arrested and several others were reported injured there throughout the day during a series of demonstrations, as members of Antifa and other leftist groups clashed with members of the far-right Proud Boys and conservatives, police said.
Local media reported that only 30 people turned up for the original protest - a "patriot prayer" rally - in Portland's Pioneer Courthouse Square on Saturday afternoon.
Disorder broke out when a group of antifascist protesters left their designated area and marched towards the Proud Boys rally, repeatedly trying to get around police blockades.
Footage showed scuffles between opposing protesters, while several antifascists were wearing face coverings or helmets, and carrying homemade shields, bats and weapons.
Eight people, including three police officers, were treated by medics at the scene, the Portland Police Bureau said. "Being attacked today on video taken by an actual journalist (because Ngo is definitely not) is the greatest thing that could have happened to his career".
Two police officers were pepper sprayed, officials said in a release.