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Ashoura Day falls on the 10th of the Islamic month of Muharram and is preceded by days of commemoration and remembrance.
One F-15 fighter plane did briefly surveil the Iranian passenger jet from a safe distance but disengaged after it established it was a civilian aircraft, the US confirmed.
Iran is one of the worst-hit countries by COVID-19.
Turnout will be seen as a key measure of support for Iran's leadership at the time of a sanctions-linked economic downturn and diplomatic isolation.
Iranian authorities confirmed two cases of the new virus on Wednesday, the first in the country. Officials later said the two patients had died.