A bus plunged into a ravine in the northern province of Mazandaran.
ISNA and Fars news agencies reported the men were stabbed to death in an ambush by assailants. Tehran was quiet on Monday as uniformed and plainclothes security forces walked the streets to quell protests over petrol prices and fuel rationing.
Top Democrat politicians are due to attend a conference organised by Muslim-American groups, but far-right activists are also planning to turn up.
Last week, Iran notified China, France, Germany, Russia and the UK of its decision to halt some commitments under the 2015 nuclear deal, a year after the US unilaterally withdrew from the accord and re-imposed sanctions.
The assault in the city of Ahvaz, which killed at least 25 and wounded more than 60 people, targeted a stand where Iranian officials had gathered to watch an annual event marking the start of Iran’s 1980-88 war with Iraq.
Iranian authorities say 41 suspects have been arrested in connection with this week's attacks in Tehran.
This makes it the second time this year Iran has implemented the strict eye for an eye penalty.
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