The demonstration was one of several planned across the country calling on Immigration Minister Jason Kenney to stop a deportation order that would see Rivera returned to the U.S. by Thursday.
Rivera, a former U.S. Army private who fled to Canada in 2007 to avoid further military service, received the order after a negative pre-removal risk assessment.
That assessment ruled she would not be in danger of punishment, torture or loss of life if deported.
Meanwhile, members of the War Resisters Support Campaign, who organized the protest, said Rivera will likely face a court martial and jail time upon her return to the U.S.
A spokesperson for Kenney said in an email that the federal government does not believe the U.S. subjects its soldiers to persecution.
Rivera has applied for Canadian permanent residency on humanitarian and compassionate grounds.
"Right now, it rests with Jason Kenney," said War Resisters Support Campaign spokesman Ken Marciniec.
"He has the power to intervene … and to let Kim Rivera and her family stay in Canada. That's what people here in Toronto and people across the country are calling for today."
Should there be no intervention, Marciniec said Rivera intends to comply with Canadian law and return to the U.S. with her husband and four children, two of whom were born in Canada.
Roughly 19,000 people signed an online petition protesting Rivera's deportation order. Nobel Peace Prize winner Archbishop Desmond Tutu and the American veterans organization Veterans for Peace have also spoken out against the planned deportation.
"The support for Iraq war resisters in Canada has been overwhelming," said Marciniec.
"We know that if this decision was up to Canadians, there's no doubt that conscientious objectors to the Iraq war like Kimberly Rivera would be allowed to stay."
Wednesday evening's rally in Toronto also attracted faith groups, local activist organizations and veteran associations.
David Milne, a member of Christian Peacemaker Teams, said he attended the protest because he had witnessed the brutality of war in three trips to Iraq.
"There were all kinds of atrocities that occurred," Milne said.
"So the fact that Kimberly Rivera deserted in protest to that shows big courage on her part and a belief in principles and human rights. It's just a no-brainer that I would come."