The demonstrations were a show of strong support for the Iranian government, even in cities that have been the source of anti-government protests in recent months and historically.
The Trump administration has said that his death was critical to disrupting "imminent attacks" on U.S. diplomatic and military personnel in the region and that the world is "safer" without him, given the hundreds of American soldiers whose deaths his Quds Force is responsible for.
Iran's Supreme Leader Ayatollah Ali Khamenei led funeral ceremonies for Soleimani at Tehran University, weeping at his casket which was draped with Iran's flag.
In the streets of Tehran, crowds joined a funeral procession and voiced anger at the U.S. strike, with many crying and calling Soleimani a national hero while holding photos and signs showing his face.
"Qassem Soleimani makes us united. And we are all emotional because of that. Trump made a big mistake. He killed our hero," said Hussein, a 24-year old student at Tehran University, choking up with emotion. "Qassem Soleimani was our father! Qassem Soleimani was our hero! They killed him!"
The number of people in the streets could not be verified. Iranian state television put it in the millions, but it seemed more likely in the hundreds of thousands, possibly over one million.
Critics of the regime overseas said the numbers don't tell the full picture, alleging that some were pressured to march or mourn.
"The media in the Islamic Republic is heavily controlled. Public gatherings are allowed only if they are pro-regime. Critics are jailed or shot," wrote Masih Alinejad, a political activist and anchor with Voice of America's Persian service. "It's not hard to use all the tools and resources of the state to stage a funeral procession."
But the emotion in the crowds was palpable. A woman, who did not provide her name, told ABC News that the Iranian people were "very proud" of Soleimani, describing him as "brave and kind" and a hero for helping lead Iranian-backed Shiite militias in the fight against ISIS in neighboring Iraq, amid Iranian fears the Sunni terror group would strike in Iran.
"We will have very hard revenge against Mr. Trump," she said.
Another young man echoed that sentiment, saying, "We would like to have retaliation," even calling for the killing of an American general.
"He has done nothing but fight terrorism his whole life," he said of Soleimani, calling him a "very popular charismatic national figure."
Soleimani was designated a terrorist by the Obama administration in 2011. As the leader of the Quds Force since 1998, he has led its fighters in operations throughout the region, supporting Syrian President Bashar al-Assad and backing Shiite militia groups in Iraq, including against U.S. troops during the Iraq War. The State Department says his forces are responsible for the deaths of over 600 American troops because of the kind of explosive devices they helped bring to Iraq.
But in Iran, he has received a martyr's funeral, and his killing has incited deep anger against the Trump administration. Along with Trump's "maximum pressure" campaign of crippling U.S. sanctions and a ban on visas for Iranian citizens, the strike has fueled anti-American sentiment, especially after Trump's promise to target cultural sites in response against any Iranian retaliation.
"Look at the crowd! They are all crying! For what?! They cannot travel in our countries. They ban us from travelling in other countries. Trump hates us! They call us terrorists! They call us a nation of terrorists! What should we do?! Are we human or not?!" said Hussein, in tears.
"We love Americans, but look at your president. He's destroying everything. He's destroying -- he's against human beings. He's destroying the human beings of every kind of humans. Our blood is not different from Americans. Why Trump is acting like this? What's wrong with us?" he added.
ABC News' Martha Raddatz and Cindy Smith reported from Tehran and Conor Finnegan from Washington.