1226 GMT February 24, 2020
The impromptu rallies, which snarled traffic in some cities including the capital Tehran, were a chance for many to breathe easily again after a tense campaign between Rouhani and his main opponent Ebrahim Raeisi.
"I'm happy and a bit relieved after a month of stress," said 27-year-old Afshin told AFP as he joined a large crowd gathered in Vali-Asr Square in central Tehran.
Across the country, young men and women, many wearing the purple of the Rouhani campaign, chanted slogans and sang together in the streets until the early hours of the morning.
Cars honked amid patriotic chants as more and more people filled the streets after dark, completely blocking traffic across wealthier north Tehran.
One group screamed with joy as a young boy threw batches of Rouhani photos into the air.
"I'm very happy because I've reached what I wanted, which was not Mr. Rouhani himself, but the path of reform, freedom and progress," said Pegah, 25.
Many were determined to ensure Rouhani now kept his vows to improve civil liberties and reform the economy.
"In the same way we campaigned for him, we will demand he keep his promises," said Afshin.
Videos on social media showed huge crowds in all four corners of the country.
"We didn't leave Mashhad; we took it back," chanted young people in the holy city of Mashhad, Raeisi’s hometown.
In the Azeri-speaking city of Tabriz in northwest Iran, crowds performed folk dances and local songs at a packed stadium as teenagers waved lighters in the air, while in the southwestern city of Ahvaz, it was the drum-heavy Bandari music that got thousands of revelers dancing.
The police were deployed to control the exuberance, and despite a few scuffles, no arrests were reported.
In Tehran's Vali-Asr Square, police tried in vain to disperse the crowds, saying they lacked a permit to gather, only to back down when the numbers became too great, and let the party continue.
Rouhani garnered 23.5 million votes out of 41.2 million ballots cast, while Raeisi bagged 15.7 million votes. Iran has 56.4 million eligible voters.
Leader of the Islamic Revolution Ayatollah Seyyed Ali Khamenei praised Iranians for their big turnout after voters queued up for hours to cast their ballots. The strong turnout of around 73% of eligible voters was "massive and epochal,” he said.