The country faced widespread protests and criticism from abroad over its handling of the incident
Iran's Supreme Leader Ayatollah Ali Khamenei has defended the country's armed forces, saying the Revolutionary Guard, who is responsible for the crash, "maintained the security" of Iran.
The supreme leader made the remarks after Iran admitted shooting down a passenger plane by mistake, reports BBC.
The country faced widespread protests and criticism from abroad over its handling of the incident.
Khamenei tried to rally support while leading Friday prayers in Tehran for the first time since 2012.
The Ukraine International Airlines Boeing 737-800 was travelling to Kyiv from Tehran on 8 January when it crashed shortly after take-off. All 176 passengers on board, including nationals from Iran, Canada, Sweden and the UK, were killed.
The ayatollah called for "national unity" and said Iran's "enemies" - a reference to Washington and its allies - had used the shooting down of the plane to overshadow the killing of senior Iranian general Qasem Soleimani in a US drone strike.
"Our enemies were as happy about the plane crash as we were sad," he said. "[They were] happy that they had found something to question the Guard and the armed forces."
The Iranian authorities initially denied responsibility but, after international pressure mounted, the hardline Revolutionary Guard admitted that the plane had been mistaken for a "cruise missile" during heightened tensions with the US.
Hours before it was shot down, and in response to the killing of Soleimani, Iranian missiles targeted two airbases in Iraq that housed US forces.
Washington initially said no US troops had been injured, but it later reported that 11 people had been treated for concussion after they showed symptoms days after the missile strikes.
Ayatollah Khamenei, 80, addressed the nation from the Mosalla mosque in the capital. The last time he did so was in 2012 on the 33rd anniversary of the country's Islamic Revolution.
Leading Friday prayers in the capital is a symbolically significant act usually reserved for times when Iran's highest authority wishes to deliver an important message, says Mehdi Khalaji of the Washington Institute for Near East Policy.
Historically, Iranian leaders have left this task to loyal clerics with strong oratorical skills, he adds.
After a meeting in London on Thursday, ministers from five nations that lost citizens in the crash demanded "full co-operation" from Iran in a transparent international inquiry.
The foreign ministers of Afghanistan, Britain, Canada, Sweden and Ukraine also said Iran must pay compensation.
Speaking on behalf of the group, Canadian Foreign Minister François-Philippe Champagne said: "We are here to pursue closure, accountability, transparency and justice for the victims - Ukrainian, Swedish, Afghan, British, Canadian as well as Iranian, through a full complete and transparent international investigation.
"In the wake of such a horrific tragedy there are many many questions. Families want answers, all of the countries assembled here today want answers, and the international community want answers. The world is waiting for those answers and we will not rest until we get them."
Mr Champagne said all those responsible must be brought to justice.
On Wednesday, Canadian investigators visited the site of the crash and examined the plane's wreckage.
They are expecting to be able to participate in the analysis of the plane's black box flight recorders - a key part of figuring out exactly what happened to the plane and how it crashed.