Clashes between hardliners and reformists have wounded four people in southern Iran as the two groups exchanged gunfire. The reformists are seeking control of parliament after making gains in the first round of voting.
The shooting broke out during the Friday vote in the city of Mamasani in southern Fars province, according to local media reports.
"Some people have shot bullets in this town without any specific targets," Hossein Zolfaghari, the interior minister's deputy for security, told the semi-official ISNA news agency.
Observers say the rare election violence underlines the tension between Iran's conservative and moderate camps.
"Safety is restored in the city now and the security forces continue looking for the identified shooters," Zolfaghari said.
Extended voting hours
Around 17 million citizens were eligible to vote in constituencies where no candidate secured the minimum 25 percent of required votes in the first round.
Voting was extended Friday in the runoff elections for 68 seats in the 290-member parliament. President Hassan Rouhani's centrist allies made big gains in the first round of voting for parliament and a clerical body. The pro-Rouhani List of Hope and the conservatives gained 103 seats in the February vote, but failed to secure a majority.
The current parliament is dominated by the hardline allies of Iran's Supreme Leader Ayatollah Ali Khamenei.
Reformists, including moderates and centrists, swept all 30 seats in the capital, Tehran, for a total of 95 across the country, while independents performed well, garnering roughly 20 percent of the contested seats.
The results of Friday's runoff are expected to be announced on Sunday, according to the interior minister. The new parliament is scheduled to begin its first session on May 27.
"We are trying to announce the final result as soon as possible. Even maybe on Saturday or Sunday," Interior Minister Abdolreza Tahmani Fazli told reporters on Friday.
A test for President Rouhani
The ballot is seen as a test for moderate President Rouhani, whose government signed off on a nuclear deal with several world powers less than a year ago.
The pro-Rouhani coalition aims to break conservative dominance in the legislature in order to push through domestic reforms. However, no one expects the vote to usher in radical changes to Iran's domestic politics, which are controlled by the clergy and Supreme Leader Khamenei, who has the final say on all state matters.
There is a chance that the incoming members of the Assembly of Experts can find a successor to Khamenei. The Supreme Leader underwent prostate surgery in 2014, and there have been recent renewed speculations about his health.
Rouhani's administration is trying to improve ties with the West, which have been strained since Iran's Islamic Revolution in 1979.