Iran's supreme leader Ayatollah Ali Khamenei is expected to lead Friday's prayers in the capital Tehran – the first time in eight years.
This comes after widespread protests against the Ukrainian airliner shot down by Iran's armed forces last week.
Iran's leadership is also under pressure over a sharp economic slowdown caused by US sanctions.
On Wednesday, President Hassan Rouhani called for national unity.
But in a rare sign of friction within the Iranian regime, Rouhani asked the military to give a full account of how he crashed the plane.
Ukraine International Airlines' Boeing 737-800, traveling to Kiev and taking many passengers en route to Canada, crashed shortly after taking off from Tehran with the loss of 176 lives aboard.
For three days, the Iranian authorities denied any responsibility, but after international pressure, revolutionary hardline guards admitted that the plane had been mistaken for a "cruise missile" during tensions with the US.
Hours earlier, the Iranian missiles targeted two air bases in Iraq, home to US forces.
This rocket attack was a response to a US drone attack in Baghdad that killed Iranian General Qasem Soleimani.
What is going on in Iran?
Iran's Mehr news agency said 80-year-old Ayatollah Khamenei would lead Muslim prayers this week at Mosalla mosque in Tehran but did not link the event to the current situation.
He quoted officials as saying "the Iranian nation will once again demonstrate its unity and magnificence."
Ayatollah Khamenei's last time leading Friday prayers in Tehran was in 2012, on the country's 33rd anniversary of the Islamic Revolution.
Friday's main prayers in the capital are a symbolically significant act, usually reserved for times when Iran's top authority wants to deliver an important message, says Mehdi Khalaji of the Washington Institute for Near Eastern Policy.
Historically, Iranian leaders have left this task to loyal clerics with strong oratory skills, he adds.
News that an Iranian missile had toppled the plane led to days of protests in some Iranian cities, particularly at universities where slogans were shouted calling the Revolutionary Guard guards and liars.
Social media footage of various funerals held for victims on Thursday showed people chanting slogans against the authorities.
Iran has arrested several people over the plane crash, and President Rouhani said the investigation would be overseen by a "special court", noting that "the whole world will be watching."
Foreign Minister Mohammad Javad Zarif acknowledged that the Iranians had been "deceived" for days, but insisted that the government be kept in the dark too.
Iranian authorities face increasing pressure on other fronts. The US economy is suffering from US sanctions and a nuclear deal signed with world powers seems almost collapsing.
Last year, violent protests broke out across the country after the government unexpectedly announced that it was rationing gasoline and raising its price.
Khamenei seems willing to defend the Revolutionary Guard
Analysis by Kasra Naji, BBC Persian
The last time Iran's Supreme Leader led Friday prayers in Tehran was at the height of the Arab Spring.
He took the opportunity to give a sermon in Arabic – something that could be heard in the Arab world. He wanted to shape events by describing developments in much of the Arab world as an Islamic Awakening. He was wrong.
It now appears that Ayatollah Ali Khamenei is willing to defend the Revolutionary Guard, which has been severely criticized in Iran for shooting down the Ukrainian plane.
There are fears that he may also ask students and protesters to crack down on his government.
Authorities called for officially sponsored national marches on Friday in support of the Revolutionary Guard. In Tehran, extensive preparations are underway to bring as many supporters as possible into another show of strength.
But many ordinary Iranians say they want him to admit that his government has only brought misery.
And the investigation?
After a meeting in London on Thursday, ministers from five countries who lost citizens in the crash demanded Iran's "full cooperation" in a transparent international investigation.
Foreign ministers from Afghanistan, Britain, Canada, Sweden and Ukraine also said Iran should pay compensation.
Speaking on behalf of the group, Canadian Foreign Minister François-Philippe Champagne said: "We are here to seek closure, accountability, transparency and justice for the victims – Ukrainian, Swedish, Afghan, British , Canadian and Iranian, through transparent international research.
"After such a horrible tragedy, there are many questions. Families want answers, all the countries here today want answers, and the international community wants answers. The world is waiting for those answers and we will not rest until they get them."
Champagne said all those responsible must be brought to justice.
On Wednesday, Canadian investigators visited the crash site and examined the wreckage of the plane.
They hope to be able to participate in the analysis of the plane's black box recorders – an essential part of finding out exactly what happened to the plane and how it crashed.