(CNN) Anti-government protesters took to the streets of Iran Saturday after Tehran admitted that it mistakenly shot down a Ukrainian passenger jet , killing 176 people.

Thousands gathered outside the gates of Amir Kabir University, near the former US embassy in Tehran, to denounce the plane crash the government blamed on human error and "US adventurism."

Ukraine International Airlines Flight 752 crashed Wednesday after takeoff from Tehran's airport. The crash came hours after Iran fired missiles at Iraqi military bases housing US troops in retaliation for a drone strike at Baghdad airport that killed Iranian commander Qasem Soleimani.

In video posted on social media, protesters chanted for Supreme Leader Ayatollah Ali Khamenei to step down and for those responsible for downing the plane to be prosecuted. "Death to the dictator," some chanted.

In one video, demonstrators chanted, "Khamenei have shame. Leave the country."

A mass vigil commemorating the victims of the doomed flight escalated into an angry demonstration following the government admission, according to Iran's semiofficial FARS News Agency. University graduates migrating to Canada were among those who perished in the crash.

FARS reported that Iranian police dispersed students who entered and blocked main roads.

Their anger was fueled in large part by the nation's armed forces claim that it targeted the passenger plane unintentionally. It attributed the crash to radar activity and fear of US action.

"The Islamic Republic of Iran deeply regrets this disastrous mistake. My thoughts and prayers go to all the mourning families," President Hassan Rouhani said.

After the missile operation in Iraq, US military flights around Iranian borders increased and Iranian military officials reported seeing aerial targets coming toward strategic centers, according to a statement by Iranian armed forces headquarters.

"The aircraft came close to a sensitive IRGC military center at an altitude and flight condition that resembled hostile targeting. Under these circumstances, the aircraft was unintentionally hit, which unfortunately resulted in death of the many Iranian and foreign nationals," the statement reads.

The victims include 82 Iranians, 63 Canadians, 11 Ukrainians, 10 Swedes, four Afghans, three Germans and three British nationals.

"Human error at time of crisis caused by US adventurism led to disaster," Iranian Foreign Minister Javad Zarif tweeted.

People stand near the wreckage after a Ukrainian plane carrying 176 passengers crashed near Imam Khomeini airport.

The commander of Iran's Islamic Revolutionary Guard Corps Aerospace Force said he had requested all commercial flights in Iran be grounded until tensions cooled off.

But the Armed Forces and government and aviation authorities chose not to do so, Brigadier-General Amir-Ali Hajizadeh said at a news conference.

Hajizadeh said the plane was misidentified as a cruise missile by an air defense operator.

The operator was unable to contact the central air defense command to confirm it. He had 10 seconds to choose between shooting it down or not, Hajizadeh said.

Hajizadeh accepted responsibility, saying that once it became clear what had happened, he thought: "I wish I was dead."

Those responsible will be prosecuted

The Ukrainian General Prosecutor's office said in a statement it was now investigating the incident as a possible case of "willful killing and aircraft destruction."

The downing of the plane was being handled as a case of "violation of traffic safety rules" and "operation of air transport resulting in death" but prosecutors said Iran's admission changed the scope to "liability for the willful killing of two or more people and the destruction of the aircraft," the statement said.

Ukrainian President Volodymyr Zelensky called Iran's admission a "step in the right direction" via Twitter.

"I insist on immediately completing identification of the bodies and their return to Ukraine," Zelensky wrote after a call with Rouhani.

"The perpetrators must be held accountable. We look forward to further legal and technical cooperation," added Zelensky, who had earlier made a call for Iran to pay compensation.

Rouhani apologized to the Ukrainian people and promised to hold those responsible "accountable," according to the readout of the conversation.

"The head of the Islamic Republic of Iran fully acknowledges that the tragedy was due to the erroneous actions of the military of this state, " according to the readout.

Rouhani in a statement released by his office earlier Saturday echoed those sentiments.

"Further investigation is needed to identify all the causes and roots of this tragedy and prosecute the perpetrators of this unforgivable mistake and inform the honorable people of Iran and the families of the victims about it," Rouhani said.

"It is also necessary to adopt the required arrangements and measures to address the weaknesses of the country's defense systems to make sure such a disaster is never repeated," he added.

Iran concedes the 'big lie' is true

Iran had previously denied US claims that the country had struck down the plane accidentally.

A US official familiar with the intelligence said the aircraft was downed by two Russian-made SA-15 surface-to-air missiles. The US saw Iranian radar signals lock onto the jetliner before it was shot down.

"No one will assume responsibility for such a big lie once it is known that the claim had been fraudulent," government spokesman Ali Rabiei said in a statement earlier Friday, according to state-run Press TV.

Iranian authorities are in possession of the two flight data recorders, also known as black boxes, which Ukrainian investigators got access to Friday. They have yet to start examining the information, but have said it included communications between the pilot and Tehran flight control.

The Canadian government was waiting for Iran to issue 10 visas to give its representatives access to the accident site, Foreign Minister Francois-Philippe Champagne said late Friday.

Two members of Canada's Transportation Safety Board and 10 Global Affairs officials were waiting in Ankara, Turkey, for the documents.

Three visas were issued for members of Canada's Standing Rapid Deployment Team (SRDT), Canadian Prime Minister Justin Trudeau told reporters.

The SRDT is a unit of specialized officials deployed to work with Canada's embassies and consulates abroad in providing critical services to Canadians during emergencies, according to the Canadian government. They were expected to arrive in Tehran on Saturday.

Rouhani told Trudeau he was committed "to collaborate, to give closure to the victims, deescalate tensions in the region and continue this dialogue," the Canadian Prime Minister said.

"What Iran has admitted to is very serious," Trudeau said. "Canada will not rest until we get the accountability, justice, and closure that the families deserve."

Trudeau said "conflict and tension" between the US and Iran "contributed" to the downing of flight PS 752.

World leaders from Germany to the United Kingdom and the United States condemned Iran.

US President Donald Trump tweeted that his administration will continue to stand with the "brave, long-suffering people of Iran."

"We are following your protests closely, and are inspired by your courage," Trump tweeted.

Death of general escalates tension

Soleimani was plotting "imminent and sinister attacks" on Americans, Trump said, adding that the action was taken to stop a war.

The Pentagon blamed Soleimani and his Quds Force for attacks on coalition bases in Iraq in recent months, including the December 27 strike that killed an American contractor and Iraqi personnel. He was also blamed for the US Embassy attack in Baghdad on December 31. The Pentagon said in addition to the hundreds killed in his time as a commander, thousands more were wounded.

Strained relations between the US and Iran have complicated the investigation. US government investigators, for example, cannot fly into Tehran and meet with government officials without a license. That prohibition is due to longstanding sanctions rather than the most recent events, according to two US officials who spoke on the condition of anonymity.

Treasury Secretary Steven Mnuchin announced Friday that Trump was authorizing yet more sanctions on Iran.