India, July 4 -- Iran prepared Friday for the days-long funeral of the late Supreme Leader Ayatollah Ali Khamenei, with banners across Tehran urging the public to rise up in support of the Islamic Republic after the devastating war that killed the 86-year-old cleric.

The country's theocracy plans to see millions flood the streets of the capital beginning Saturday in scenes reminiscent of the burial of the late Supreme Leader Ayatollah Ruhollah Khomeini in 1989. That could provide a boost for Iran's government, particularly as it tries to leverage its hold on the Strait of Hormuz in negotiations with the United States over a permanent end to the war, and as concern still lingers that Israel could attack yet again.

Despite that, a powerfu...