Video captured by The Lantern showed the man being hit from behind inside the Ohio Union in Columbus, Ohio.
The protesters, using a bullhorn, said to the large crowd, "We don't have to compromise."
At the same time, a man in a scarlet and gray long-sleeved shirt with a baseball hat was seen sprinting down the stairs and shouting, "You idiot!"
Video showed the man smashing into the back of the protester, sending to protester falling down the stairs and onto the ground as the crowd gasped.
A man in a Guy Fawkes mask was seen jumping forward from the crowd to hit the attacker as police officers moved in to take the tackler away.
The crowd chanted "shame" as police led the suspect away.
Protest have sparked across the nation following President-elect Trump's victory over Hillary Clinton on Election Day.
Hundreds of students marched through the streets of Los Angeles on Monday calling for unity following the election results.
Over the weekend, approximately 8,000 demonstrators gathered in downtown Los Angeles to form the largest anti-Trump rally to date since Election Day.