Iranian authorities fired live ammunition to disperse protesters in Tehran on Sunday night, wounding several people, according to witness accounts provided to the Guardian and footage circulating on social media.

Hundreds of protesters had defied a heavy security presence in the Iranian capital to hold vigils and demonstrations after the government belatedly admitted its own forces had shot down a Ukraine International Airlines passenger jet last Wednesday, killing all 176 people on board.

There were fresh protests on Monday in at least two universities in the country. Though campus protests are not unusual in Iran, they come during a period of extraordinary tumult in the Islamic Republic, with an economy suffocated by US sanctions, the largest protests in the regime’s history crushed by violent force in November, and popular revulsion that the country’s armed forces shot down a jet loaded with Iranian citizens – then denied doing so.

A government spokesman on Monday denied there had been any cover-up: “Some officials were even accused of lying and a cover-up but, in all honesty, that was not the case,” said Ali Rabiei, adding that all details provided by officials before Saturday’s revelation had been based on the information they had.

“All of those who expressed opinions on those days, at the peak of America’s psychological war ... did so based on existing information at the time.”

Analysts said the renewed anti-government protests were still concentrated among the restive middle classes and students and not an existential threat to the government. But the backlash had wiped out any momentum the government had hoped to gain from the outpouring of grief and rage that followed the killing of Gen Qassem Suleimani by a US drone strike on 3 January, and Iran’s missile strikes on American forces in Iraq five days later.

“It was thought they would have gotten a break, but they have … ruined it for themselves,” said Dina Esfandiary, an Iran specialist at the Century Foundation thinktank.

A witness told the Guardian that groups, many led by women, gathered in Tehran’s central Azadi Square on Sunday evening wearing masks and scarves to hide their identities, confronting riot police and officers in plain clothes.

Play Video 0:50 Protesters wounded during demonstrations in Iran over downing of plane – video

“One of [the officers] with a white beard was filming regular people and smiling flagrantly while we were shouting in their faces,” Ronak, 35, said. “Thirty minutes later, the shooting of teargas started and the crowd was shouting in between cries and coughs: ‘For so many years of crimes, down with this wilayat [Iran’s theocratic system of government].’”

She said the crowd rushed away from the teargas but continued chanting, and security forces started to fire “compact and hard black bullets”, a form of non-lethal ammunition. “[They fired] constantly, without stop,” she said.

“People were on the ground. One of the teargas canisters was fired near us. My friend was shot by paintball bullets in his head. For a few seconds I thought he had lost his eye, but the bullet hit his eyebrows. Blood was dripping from his eyebrows. His face was wet and he was constantly coughing.”

The crowd dispersed after security forces began “shooting bullets that were aimed at heads”, she said.

“The crackdown was too intense, so our comrades dispersed,” she said.

The witness’s account could not be independently confirmed, but accorded with another witness’s report that security forces initially fired teargas to disperse the crowds and then started firing bullets. “It was a very bad situation,” the woman, who asked not to be identified, said in a message provided to the Iranian activist Masih Alinejad.

“They were firing teargas repeatedly. We couldn’t see anywhere and we were screaming. We were getting blinded. Forces were firing teargas back to back. A young girl beside me was shot in the leg. It was terrible, terrible.”

She provided a video from near Azadi Square showing bloodstains along the pavement, one of several similar videos being circulated by Iranian activists on Sunday evening and Monday morning. “It’s the blood of our people,” a woman said in one clip.

Another video purportedly from near Azadi Square shows several people appearing to be wounded on the ground including a woman lying on a bloodied pavement. “They shot her with a bullet,” a man says.

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Tehran’s police chief said in a statement on Monday that no shots were fired during Sunday’s events and that officers were under orders to show restraint. “At protests, police absolutely did not shoot because the capital’s police officers have been given orders to show restraint,” Hossein Rahimi said.

There were demonstrations in several other cities but it was unclear whether similar force had been used to disperse those. A protester in the southern city of Shiraz said demonstrations there were relatively small and short. “People were sad and they lit some candles and shouted slogans,” said Mehdi, 49. “But most were middle-class and so didn’t do any radical acts. They were dispersed by police after two hours.”

Iran’s revelation that it shot down the jet sparked anger and disgust even among ordinarily pro-regime figures including several journalists. A former news anchor with a state-run TV2 channel said on Monday she would never return to the network, writing in an Instagram post: “Forgive me for the 13 years I told you lies.”

The incident has also led to unprecedented statements of contrition from Iran’s hardline Revolutionary Guards. Ali Hajizadeh, the organisation’s aerospace commander, said at the weekend that an air defence unit near Tehran’s airport had been advised a cruise missile had been fired at the country and was on its highest alert.

He said the unit first observed the passenger jet around 19 miles away and a missile operator had identified it as a cruise missile. Due to a breakdown in his communications system, the operator was unable to seek approval to fire at the cruise missile.

“He had 10 seconds to decide: he could hit or not hit [the target],” Hajizadeh said. “Under such circumstances, he decided to make that bad decision: he engages, the missile is fired, and the plane is hit.”