In an election upset in Iran, the reformist candidate, Masoud Pezeshkian, who advocated for moderate policies at home and improved relations with the West, won the presidential runoff election, beating his hard-line rival, the Ministry of Interior said on Saturday morning.

Mr. Pezeshkian, 69, a cardiac surgeon, got 16.3 million votes to defeat the hard-line candidate, Saeed Jalili, delivering a blow to the conservative faction and a major victory for the reformist faction that had been sidelined from politics for the past few years. Mr. Jalili received 13.5 million votes.

After polls closed at midnight, turnout stood at 50 percent, about 10 percentage points higher than in the first round of the election with about 30.5 million ballots cast in total, according to Iran’s interior ministry. The first round saw a record-low turnout because many Iranians had boycotted the vote as an act of protest.

However, the prospect of a hard-line administration that would double down on strict social rules, including enforcing mandatory hijab on women, and remain defiant in negotiations to lift sanctions, apparently spurred Iranians to turn up at the polls in slightly larger numbers.

Mr. Pezeshkian’s supporters took to the streets in the predawn hours of Saturday, according to video footage on social media and his campaign, honking horns, dancing and cheering outside his campaign offices in many cities, including his hometown, Tabriz, when initial results showed he was leading. They also took to social media to congratulate Iranians for turning up at polls to “save Iran,” a campaign slogan of Mr. Pezeshkian’s.

“The end of the rule of minority over majority. Congratulations for the victory of wisdom over ignorance,” Ali Akbar Behmanesh, a reformist politician and head of Mr. Pezeshkian’s campaign in the province of Mazandaran, said in a post on X, formerly Twitter.