One pledged he would confront Iran’s enemies, the other vowed to make peace with the world. One intends to double down on social restrictions, the other promises to ease stifling rules for young people and women. One identifies as an Islamic ideologue, the other as a pragmatic reformist.

Iranians were voting for the country’s next president on Friday in a race that has turned into a fierce competition and where, for the first time in more than a decade, the outcome is difficult to predict.

The runoff on Friday, between the ultraconservative Saeed Jalili and reformist the Dr. Masoud Pezeshkian, is taking place after a general election last week failed to produce a candidate with the required 50 percent of the vote.

The result may hinge on how many Iranians who sat out the vote in the general election decide to participate in the runoff. Turnout was at a record low of 40 percent last week, with the majority of Iranians boycotting the vote out of anger at the government or alienation and apathy over the failure of previous governments to produce meaningful changes.

Voters face a choice between two starkly different outlooks on how to govern the country as it faces a multitude of challenges at home and abroad. The two candidates represent polar ends of the political spectrum: Mr. Jalili is a hard-liner known for his dogmatic ideas, while Dr. Pezeshkian has gained traction among voters by calling for moderation in both foreign and domestic policy.

Mr. Jalili rejects any accommodation with the West, saying Iran should build its economy by expanding ties with other countries, mainly Russia and China. A former nuclear negotiator, he opposed the 2015 nuclear deal for making too many concessions and supports the mandatory hijab law for women and restrictions on the internet and social media

Mr. Pezeshkian has vowed to reinvigorate the economy by negotiating with the West to remove sanctions. He has promised to abolish the morality police, who enforce the hijab law, and also to lift internet restrictions and rely on technocrats to run the country.

“This election is about competing currents, it’s not about competing candidates per se,” said Sanam Vakil, the Middle East director for Chatham House. “The currents reflect an attempt at preserving revolutionary values, the Islamic ideology and the notion of resistance within the Iranian state versus an alternative that isn’t quite reform but a more moderate and open social and political climate.”

In Iran’s theocratic system of governance, the president does not have the power to upend major policies that could lead to the kind of change that many Iranians would like to see. That power resides in the person of the supreme leader, Ayatollah Ali Khamenei. Two previous presidents who were elected in landslides pledged changes but failed to deliver, leading to widespread disillusionment.

Nevertheless, the president is not entirely powerless, analysts say. The president is responsible for setting the domestic agenda, choosing the members of the cabinet and even exercising some influence in foreign policy.

Mr. Khamenei voted early on Friday morning at the religious center attached to his compound, state television showed. He cast his ballot in a box placed on a lone table in a big hallway and waved.

“At this stage people should naturally be more resolved and finish the job,” Mr. Khamenei said. He gave no indication of which candidate he supported.

Polling stations opened on Friday at 8 a.m. and are scheduled to close at 10 p.m., although an extension is likely. Many Iranians vote in the evening because of the summer heat.

Mr. Khamenei said on Wednesday that he was disappointed by the low turnout in the first round of voting, and acknowledged some disenchantment with Islamic rule. But he dismissed efforts to equate low voter turnout with a rejection of the system and called on people to vote.

“We have said this repeatedly,” he said. “People’s participation is a support for the Islamic Republic system, it is a source of honor, it is a source of pride.”

With the runoff, turnout was expected to be slightly higher because of the stark polarization, but also because many people fear the potential for an extreme hard-line administration. The Interior Ministry said representatives from both candidates would be present at polling stations during voting and ballot counting.

Mr. Jalili, is part of a fringe but influential hard-line political party known as Paydari with followers that look up to him more as an ideological leader than a politician. Dr. Pezeshkian, a cardiologist and former health minister and member of Parliament, was until recently not widely known outside of political and health circles.

Their lineup of advisers and campaign staff reflects the stark differences in their policies and has given voters a glimpse into what each administration might look like.

Mr. Jalili’s team includes conservative hard-liners who pledge that his presidency would be a continuation of the “resistance policies” of former President Ebrahim Raisi, whose death in a helicopter crash in May prompted an emergency election. Military commanders and senior clerics have endorsed him, praising his zealotry in religious and revolutionary matters.

Dr. Pezeshkian has assembled a team of seasoned technocrats, diplomats and ministers, including the former foreign minister, Mohammad Javad Zarif, who are trekking the country in support of him mostly by warning of doomsday if Mr. Jalili is elected.

“The election on Friday, July 5 is about the future,” Mr. Zarif said on Tuesday, speaking in a virtual town hall on the social media app Club House, where thousands of Iranians have gathered every night to discuss the election. “In reality we have a referendum. These two choices are as different as day and night,”

Reformists are counting on measurable defections from the conservative camp, where Mr. Jalili has long been a divisive figure. Many conservatives consider him too extreme, analysts say, and fear his presidency would deepen the rupture between the government and the public and put Iran on a collision course with the West.

Polls conducted by government agencies seemed to indicate that a sizable number of voters who supported the more moderate conservative candidate, Mohammad Baqer Ghalibaf, the speaker of the Parliament, would flock to Dr. Pezeshkian in an effort to block Mr. Jalili’s chances for the presidency.

Many Iranians are still resolved to boycott the vote. Mahsa, a 34-year-old accountant in Isfahan, said she would not cast a ballot and was not buying the logic that she had to pick between “bad and worse.”

But others said in interviews and on social media that they were having a change of heart, mostly because they were terrified of Mr. Jalili’s ascent.

Babak, a 37-year-old businessman in Tehran who asked that his last name be withheld out of fear of retribution, said he and family members would break their boycott and vote for Dr. Pezeshkian. “We kept going back and forth on what to do, and at the end we decided we must try to stop Jalili, otherwise we will suffer more,” he said.

A prominent political activist who had not voted in the first round, Keyvan Samimi, said in a video message posted on social media from Tehran that he had decided to back Dr. Pezeshkian. “We are casting a protest vote to save Iran,” he said. The frenzy against Mr. Jalili has intensified as the vote has drawn near. Prominent political figures compared him to the Taliban and accused him of running a “shadow government.”

Mr. Jalili’s supporters pushed back, accusing the reformists of name-calling and fear mongering. They counterattacked by characterizing Dr. Pezeshkian as a puppet of the former moderate president, Hassan Rouhani. They have said the doctor lacks a real plan and was overreaching on issues that would fall outside his authority as president — particularly his promise to abolish the widely detested morality police and normalize ties with the United States.

Reza Salehi, 42, a conservative who works in public relations and campaigned for Mr. Jalili, said in an interview from Tehran that “Mr. Jalili is absolutely not dogmatic.” He added that the candidate was better prepared to govern and that the so-called shadow government was more similar to a think-tank and not the sinister plot that his rivals claimed.

Analysts say the outcome of the runoff election remains hard to predict. Dr. Pezeshkian may have been allowed to run as a token reformist candidate to increase participation, some say, but he has at least turned into a wild card.

“The two candidates are running neck and neck and it’s not clear whose name will come out of the ballot box,” said Nasser Imani, a political analyst in Tehran, in a phone interview. “What’s certain is that in this election, saying ‘No’ is the trend. No to the election or no to this candidate, no to that candidate.”