Iran’s Supreme Leader Ayatollah Ali Khamenei is to lead prayers in Tehran’s capital on Friday-the first time in eight years.
This comes in the wake of massive angry demonstrations over last week’s shot down Ukrainian passenger plane by Iran’s army.
Iran’s government is also under pressure from a sharp economic downturn brought about by US sanctions.
President Hassan Rouhani invoked national unity on Wednesday.
But Mr Rouhani called on the military to give a full account of how it shot down the plane in a rare sign of discord within the Iranian regime.
Although flying to Kyiv and carrying several passengers on their way to Canada, the Ukraine International Airlines Boeing 737-800 crashed shortly after taking off from Tehran with the loss of all 176 lives on board.
Iranian officials denied any blame for three days but the hardline Revolutionary Guards acknowledged after international pressure that the plane had been mistaken for a “cruise missile” amid heightened tensions with the US.
Hours before, two airbases in Iraq had been attacked by Iranian missiles which housed US forces.
The rocket attack followed a US drone strike in Baghdad the killed senior Iranian general Qassem Soleimani.