The veteran, Mike Hastie, was standing several feet from a line of federal troops and shouting at them about his time in Vietnam when a troop rushed him from his side and sprayed pepper spray in his face.
Mr Hastie was rebuking the troops for following orders he believes are unlawful rather than upholding an oath to the US Constitution.
He continued yelling at the federal troops as they walked away despite having the pepper spray in his eyes. Eventually he breaks off his pursuit and wipes the spray out of his eyes.
"That was a direct spray," Mr Hastie says in the video.
The encounter was captured by journalist Andrew Kimmel, who has been on the ground in Portland covering the protest.
Mr Hastie is a member of the activist organisation Veterans for Peace.
The protests have only intensified since the federal troops arrived in Portland and began using disproportionate force against protesters.
Last week, United Nations spokesperson Elizabeth Throssell said the organisation has received reports that US protesters were being detained without reason and being beaten by police.
"There have been reports that peaceful protesters have been detained by unidentified police officers and that is a worry because it may place those detained outside the protection of the law and may give rise to arbitrary detention and other human rights violations," Ms Throssell said during a press conference.
Ted Wheeler, the Mayor of Portland, has demanded Mr Trump remove the federal troops from his city. While visiting with protesters, the mayor was tear gassed by federal troops.
"We are standing firmly with our non-violent protesters," Mr Wheeler said during a CNN interview.
More than two dozen Senate Democrats sent Mr Trump a letter demanding he provide an explanation for sending federal troops into the city.
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