Demonstrators in Tehran chanted "Down with America" and "Down with Israel" as they held up photographs of the slain general.
Attending the rally at the capital's Imam Khomeini Grand Mosalla mosque, President Masoud Pezeshkian vowed to "neutralise the enemy's sinister schemes to create discord among Muslims."
"We will stand on the side of the truth. We will continue on the path of martyr Soleimani with strength and defeat these cowards," he told the crowd.
Commemorations were also held in other cities, including Soleimani's hometown Kerman, state television images showed.
Soleimani was seen as the key sponsor of the so-called "axis of resistance", an alliance of Middle East militant groups that Iran supported to counter Israel and its US ally but which suffered heavy blows last year.
He was killed in a US drone strike at Baghdad airport on January 3, 2020 on orders from then-president Donald Trump, who is about to return to office for a second term.
Soleimani was a popular figure in Iran and foreign ministry spokesperson Esmaeil Baqaei called him "commander of hearts" in a post on social media platform X.
"Great commanders are not conquerors of known territories in geography, they are conquerors of hearts, and that is why their conquests remain forever," Baqaei said.
The "axis of resistance" fostered by Soleimani took heavy blows during 2024.
The rule of the Assad clan in Syria, Iran allies for nearly 45 years, collapsed in the face of a lightning offensive by Islamist-led rebels, while both Hamas and Lebanon's Hezbollah suffered heavy losses at the hands of the Israeli military.
A memorial service for Soleimani in Kerman in January 2024 was the scene of Iran's deadliest attack since 1978. Twin bombs claimed by the Islamic State jihadist group killed more than 90 people and wounded hundreds.
