The protesters pitched several red tents outside the Langevin building, across from Parliament Hill, to illustrate the urgency of the country's affordable housing shortage and the plight of the homeless.
Spokesman Francois Saillant urged Harper "not to sacrifice contributions to social housing" by ending long-term federal subsidies that currently fund 600,000 social housing units across the country.
The group, known as Front d'action populaire en reamenagement urbain (FRAPRU), wants the federal government to commit to continuing those subsidies.
As many as 200,000 people in Canada a year grapple with homelessness.
Pointing to Canada Mortgage and Housing Corporation figures, the group says the number of social housing units subsidized by the federal government has fallen by 34,000 since Harper was elected in 2006.
In this year's budget, however, the government surprised anti-poverty advocates by announcing a five-year renewal of funding for the Homelessness Partnering Strategy.
The budget cited evidence from a massive pilot project, run by the Mental Health Commission of Canada, that helped find and pay for homes for mentally ill homeless people in five cities across Canada. The pilot also provided recipients with as many social services they needed to stay housed.