The call came as an aboriginal elder in northern Manitoba embarked Tuesday on a hunger strike of his own.
In his letter to Harper, NDP Leader Tom Mulcair called on the prime minister and the Governor General to meet with aboriginal leaders to end the protests.
Chief Theresa Spence of northern Ontario's remote Attawapiskat First Nation stopped eating a week ago, vowing to die unless the government started showing more respect for aboriginal treaties.
"I ask that you please act swiftly to avoid a personal tragedy for Chief Spence," Mulcair wrote.
"I look forward to your early positive response to this urgent matter."
Spence has been living in a tipi on Victoria Island in the Ottawa River, less than a kilometre from Parliament Hill, since beginning her fast Dec. 11.
Victoria Island is considered by the Anishinabe as traditional territory. New Democrat MP Paul Dewar visited Spence on the island Tuesday and reported that she is so far in good health.
Last week, Aboriginal Affairs Minister John Duncan proposed a meeting with Spence to discuss issues affecting Attawapiskat, but there was no response.
Harper met with Assembly of First Nations Chief Shawn Atleo as recently as Nov. 28 to review progress the government has been making in addressing aboriginal issues, said spokeswoman Julie Vaux.
"The prime minister hosted an historic gathering of the Crown and First Nations this past January," Vaux said in an email.
"Since then, the government has been working with First Nations leadership to make progress in several areas, most notably education and infrastructure on reserve."
The Liberals and the Assembly of First Nations also sent letters Tuesday to Harper and Gov. Gen. David Johnston, calling for an urgent meeting to discuss Spence's demands.
"I urge you to agree to participate in this meeting and meet with Chief Theresa Spence to hear directly from her why she has felt it necessary to take such drastic action," wrote Liberal aboriginal affairs critic Carolyn Bennett.
Spence's hunger strike has become a cause celebre for some First Nations activists, prompting widening aboriginal protests across the country.
Attawapiskat was in the spotlight last winter when the reserve's severe housing crisis made international headlines.
More recent protests, however, have centred around aboriginal concerns with the Harper government's omnibus budget legislation, Bill C-45.
Supporters of a growing grassroots movement known as Idle No More have complained that the Harper government's policies have directly — and negatively — impacted aboriginal communities and the environment at large.
They want First Nations to be recognized as sovereign stakeholders in decisions affecting the country's land and resources.
Elder Raymond Robinson of the Cross Lake First Nation in Northern Manitoba launched his own hunger strike Tuesday, with the support of the AFN's Atleo and other aboriginal leaders.
"Now more than ever, we must see immediate and urgent attention and concrete commitments by government to work together with First Nations to address the unfulfilled promises, commitments and agreements that leave First Nations people struggling to meet the basic standards of life on a daily basis," Atleo said in a statement issued after a meeting of leaders in Cross Lake.
"First Nations stand in solidarity and will not back down. The time is now — and this must be met with urgent and priority attention by government, there are lives at stake."
Also on HuffPost