Hardline candidate Saeed Jalili and reformist Massoud Pezeshkian are neck and neck in Iran's presidential election.
Both candidates have been hovering around 40% of the vote with more than eight million ballots already counted, with each overtaking the other in recent hours.
The election will go to a second round - set for next Friday - if neither Mr Jalili or Mr Pezeshkian win 50% of the vote.
Two security force members were killed after unidentified gunmen attacked a vehicle carrying election boxes in Sistan-Baluchestan province, according to state media reports.
Mr Pezeshkian, a former heart surgeon and health minister, has promised a different approach, saying the actions of the morality police, who enforce strict dress codes on women, are "immoral".
The vote is to replace former president Ebrahim Raisi, who died on 19 May when the helicopter he was in crashed into a mountain, with seven other people also killed.
Though there are 61.5 million eligible voters in Iran, turnout is expected to be low for this election. It hit record lows in parliamentary elections in March and the last presidential election in 2021.
Earlier unofficial estimates put the turnout to just under 40 per cent of all those eligible to vote - which if confirmed show the lowest turnout in a presidential race in Iran since the 1979 Islamic revolution.
Supreme leader Ayatollah Ali Khamenei, who is the ultimate authority in the country, has called for “maximum” turnout.
Iran was shaken by a huge wave of protests in 2022 following the death of 22-year-old Mahsa Amini, who was detained by the morality police for allegedly violating Iran's strict dress code.
Human rights groups say hundreds were killed in the crackdown and thousands detained.
