The election of a moderate Iranian president could help rein in hostility between Tehran and its Arab neighbours, but many Arabs doubt he can end a sectarian confrontation that has been inflamed by war in Syria.
Hassan Rohani, a Shi'ite cleric known for a conciliatory approach and backed by reformists, will have only limited say in policy determined by Iran's supreme leader; but with the Syrian carnage fuelling rage among Sunni Arabs across the region, any gestures from Tehran may help contain it.
"We hope the new Iranian president will be a believer in a political solution in Syria," said one ambassador at the Arab League in Cairo. "All that we read about Rohani might be grounds for hope - but there is a great difference between election campaigns and what is said once in office."
For the United States and Western powers, at odds with Iran for decades and now rallying with arms behind rebels fighting Syria's Iranian-backed president, fierce religious enmities in the oil-rich Middle East add to fears of wider instability.
In Saudi Arabia, whose US-allied rulers lead opposition to what they see as Iran's drive to spread its power and religion, well-informed analyst Jamal Khashoggi said: "I'm sure for the Saudi leadership this is the best outcome of the elections."
He recalled that Iran's last reformist president, Mohammad Khatami, who visited Riyadh while in office from 1997-2005, had mended ties - but at a time of less ferocious disputes. Unlike now, Khashoggi said, "Iran was not meddling heavily in Syria, Bahrain and Yemen ... There were no Shi'ites killing Sunnis."
In Syria, where mainly Sunni rebels are battling Iran's ally President Bashar al-Assad and his Alawite establishment, who belong to an offshoot of Shi'ism, opposition activists saw little hope for change from Rohani:
"The election is cosmetic," said Omar al-Hariri from Deraa, where the uprising began during the Arab Spring two years ago.
Muhammed al-Husseini, from the Sunni Islamist rebel group Ahrar al-Sham in Raqaa, noted power in Iran rested with Supreme Leader Ayatollah Ali Khamenei. "The powers given to the Iranian president are weak these days," he said. "They are fake powers."
In Bahrain, whose Saudi-backed Sunni monarchy accuses Iran of fomenting protests among the Shi'ite majority on the island since 2011, Information Minister Samira Rajab told Reuters: "I think Rohani is one of a team. And anybody who comes from that team will continue the same policy ... We have no more trust in the Iranian regime after what happened in Bahrain."
In Egypt, by far the biggest Arab nation, new rulers from the Muslim Brotherhood had lately launched a rapprochement with Iran but have now joined a Sunni call for jihad in Syria after Iran's Lebanese ally Hezbollah sent in its fighters last month.
Traditionally more open than the Saudi clerical hierarchy to conciliation across the sectarian divide, the Brotherhood still hopes for a change of heart in Tehran: "We are looking forward to seeing how the winner is going to act," said Murad Ali, a spokesman for the Islamist movement's Freedom and Justice Party.
"Will there be any change to the policies from the Iranians, especially concerning the Syrian crisis? We are in general open to cooperation with Iran ... However, we do have our concerns ... related to ... their interference in Syrian affairs."
On the streets of Cairo, however, sectarian passions are running high, piling pressure on Egyptian and other Arab rulers.
Outside the Al-Azhar Mosque, built 1,000 years ago by the Shi'ite Fatimid caliphs who founded the city but now a major seat of Sunni learning, construction worker Mohamed Abdelsattar, 35, said: "All Egyptians hate Iran after what has happened in Syria. What's happening there now is Shi'ites killing Sunnis."
Limousine driver Abdelaziz Darwish, 57, had low expectations of any change in Tehran: "All Iranians are the same," he said. "Shi'ites are more dangerous even than the Jews."
Standing by his fresh-juice stand, Khaled Fathi, 49, twinned his anger at Iranian involvement in Syria with suspicion of the welcome that Islamist President Mohamed Mursi gave earlier this year to Iran's hardline outgoing president, Mahmoud Ahmadinejad:
"Iran makes problems for us all over the world," he said. "Iran is helping Mursi, I'm sure of it."
A group of Lebanese Sunni clerics, visiting Al-Azhar while attending the Cairo conference that has issued a call for holy war in Syria, voiced some hope for change from Rohani, however:
"Maybe this new president in Iran will be better," said Sheikh Hassan Abdelrahman from the city of Tripoli, which has seen recent fighting between Lebanese Sunnis and Shi'ites.
Sheikh Malik al-Jdeideh, also from Tripoli, said: "We came to Egypt to tell Mohamed Mursi that we reject Iranian actions in Syria ... But we are working for all religions to be at peace."
Sectarian atrocities in Syria, and the open appearance of Iran's Lebanese allies on the battlefield, has forged an unusual degree of unity among major Arab governments following the wave of revolt that shook the region and notably replaced US ally Hosni Mubarak in Egypt with the Islamists of the Brotherhood.
Iran's new influence in Iraq - after the 2003 US invasion replaced the Sunni Saddam Hussein with an elected, Shi'ite-led government - had already put Saudi Arabia on the defensive. And Tehran's nuclear dispute with the West and Israel has alarmed oil-exporting neighbours, who fear a war, with all the upheaval it would bring.
One Arab League ambassador said Gulf states hoped Rohani, a former nuclear negotiator, might help to defuse that tension.
But a Gulf envoy at the League said Rohani would have little power and was unlikely, in any case, to differ in his views: "They all aim to export the Iranian revolution to neighbouring states and interfere in the Gulf states and Syria and Lebanon."
For Shi'ites who live in Sunni-ruled states, and often complain of being unfairly branded as agents of the Persian-speaking power, any reduction in tension would be welcome.
Khalil Ebrahim al-Marzooq of Bahrain's opposition al-Wefaq party, which speaks for many Shi'ites, said the election might bring warmer ties across the Gulf that would help his community.
"When relations are better," he said, "it gives the government no excuses to deprive the people of Bahrain of their rights."
Jafar al-Shayab, a former elected official in the mainly Shi'ite Saudi district of Qatif, said: "If this sectarian war going on in the region can cool down or stabilise, that will help to improve the relations between the Sunnis and the Shi'ites here."
Khalil al-Anani, a senior fellow at Washington's Middle East Institute currently in Cairo, said Rohani's ability to induce the Iranian leadership to take the heat out of its standoff with the Sunni Arab powers was unclear, but of vital importance.
"Mending Iran's relations with Arabs would require Rohani to secure strong support from other influential power centres in Iran ... which is unlikely in the short term," he said.
"The question of whether Rohani can be another Khatami is important and crucial for both Iran and the Arabs."