Iran denied a "cover-up" 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 brought down by a missile shortly after taking off Wednesday from Tehran, killing all 176 passengers and crew on board.
The Kiev-bound plane was knocked out of the sky hours after Tehran had launched a wave of missiles at US troops in Iraq in retaliation for the killing of a top Iranian general.
The Islamic republic initially denied Western claims based on US intelligence that the airliner had been struck by a missile before acknowledging it on Saturday.
Its handling of the matter saw a memorial 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 online, but it was difficult to assess how many people attended.
"I don't know why they didn't cancel the flights that night," a Tehrani named Hamid told AFP.
Protesters chanted "Death to dictator" and against the Revolutionary Guards, Fars news agency said, a rare move for a country where media usually refer to demonstrators as "rioters" and refrain from publishing such slogans.
Internet monitor NetBlocks reported a drop in connectivity Monday at Tehran's Sharif University ahead of any new demonstrations.
"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."
The spokesman said 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."
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 Sunday night's gathering at iconic Azadi Square south of the city centre.
"The police treated the people who had gathered with patience and tolerance," 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 Rouhani 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. Others were Ukrainians, Afghans, Britons, seven Swedes and 10 people who resided in the Scandinavian country.