TEHRAN — Iran denied a "cover-up" on Monday after taking days to reveal an airliner was accidentally shot down last week, a disaster that sparked demonstrations and calls for a fully transparent investigation.

The Ukraine International Airlines Boeing 737 was shot down by a missile shortly after taking off from Tehran before dawn on Wednesday, killing all 176 passengers and crew on board.

The Kiev-bound airliner was knocked out of the sky hours after Tehran had launched a wave of missiles at Iraqi bases where US troops are stationed in retaliation for the killing of a top Iranian general.

Authorities in the Islamic republic initially denied Western claims based on US intelligence that the passenger plane had been struck by a missile before acknowledging it on Saturday.

Their handling of the matter saw a memorial gathering by students at a Tehran university turn into a demonstration on Saturday evening before it was dispersed by police.

Demonstrations were also held in the capital on Sunday night, according to unverified videos shared on social media, but it was difficult to assess how many people took part.

"In these sorrowful days, many criticisms were directed at relevant officials and authorities," said government spokesman Ali Rabiei.

"Some officials were even accused of lying and a cover-up but, in all honesty, that was not the case," he said.

"Lying is intentionally and knowingly faking the truth. Lying is covering up. Lying is knowing a fact and not expressing it or twisting the truth."

The spokesman said all details provided by officials prior to Saturday's revelation had been based on the information available to them at the time.

"All of those who expressed opinions on those days, at the peak of America's psychological war against the Iranian nation... did so based on existing information at the time."

Germany called on Iran to allow people to show their grief and "protest peacefully and freely".

Its foreign ministry spokeswoman Maria Adebahr described as "very worrying" videos reportedly showing security forces cracking down on demonstrators.

Tehran's police chief said officers had been ordered to show "restraint" after the latest gathering on Sunday night at iconic Azadi Square south of the city center.

"The police treated the people who had gathered with patience and tolerance" at the demonstrations, said General Hossein Rahimi.

"The police did not shoot at the gatherings at all because a restraint order (had been issued) for police in the capital."

President Hassan Rohani promised a "thorough investigation" into the disaster in a phone call with Sweden's Prime Minister Stefan Lofven, his office said.

The majority of those on Flight PS752 were Iranians and Canadians, including dual nationals. Other victims were Ukrainians, Afghans, Britons and seven Swedes as well as 10 people who reside in the Scandinavian country.

"We must strive to ensure that such a shocking incident is not repeated anywhere in the world," Rohani said.

The president noted the disaster occurred at a time of heightened tensions in the region after a US drone strike killed Iranian general Qasem Soleimani in Baghdad on Jan. 3.

"We must all join hands to bring security back to the region and allow peace to prevail," he added.

Iran has come under mounting international pressure to ensure its investigation into the tragedy is full and transparent.

Prime Minister Justin Trudeau told a memorial event in Edmonton for the 57 Canadians who lost their lives that "this tragedy struck our Iranian-Canadian community".

"We want to assure all families and all Canadians that we will not rest until there are answers," the Canadian leader said. "We will not rest until there is justice and accountability."

Iran has invited experts from Canada, France, Ukraine and the United States to take part in the investigation.

Despite footage from the site of disaster appearing to show bulldozers clearing the area, the Revolutionary Guards' top commander denied any evidence had been tampered with.

"We didn't touch anything," said Major General Hossein Salami.

"We didn't move the wreckage of the aircraft, we didn't change the scene, we didn't move the air defense system, and we didn't (alter) the radar readings," he told parliament.

On the diplomatic front, Britain summoned Iran's ambassador to London after its Tehran envoy was briefly arrested before being called before Iran's foreign ministry for allegedly attending the demonstration.

Prime Minister Boris Johnson's spokesman said Britain would convey its "strong objections" over Rob Macaire's arrest, calling it an "unacceptable breach" of diplomatic protocol. — AFP