Residents of Iraq's second city, scarred by years of jihadist rule, demanded justice as Prime Minister Adel Abdel Mahdi decreed three days of national mourning following Thursday's accident.

Mosulites had resumed the annual festivities this year for the first time since Iraqi forces ousted the Islamic State group following years of brutal jihadist rule.

But the celebrations turned to tragedy when the overcrowded vessel ran into trouble as it headed to a popular picnic site across the River Tigris.

Mosul security officials blamed the accident on high water levels and overcrowding on the boat.

Hundreds of relatives of victims and residents gathered Friday at the scene of the accident, where prayers were held for the dead.

Many said the disaster could have been avoided, and chanted "corruption is killing us!"

"We want those responsible to be brought to justice," said Mohammed Adel, 27, whose father was among those who died.

He accused officials of failing to enforce safety standards.

Abu Salem, who lost his wife and two children in the accident which also coincided with Mother's Day in Iraq, pinned blame for the tragedy on profit motives and the corruption that is endemic in Iraq.

The managers of the picnic site were "criminals... I want them to pay up and I won't leave here until they do", the 50-year Iraqi said.

Hundreds of demonstrators later gathered at the city's amusement park to stage an impromptu protest against "the thieves" running the city.

Reflecting the popular mood, Grand Ayatollah Ali Sistani, spiritual leader of Iraq's Shiite Muslim majority, in his Friday sermon accused "the authorities in charge of supervision of not doing their job".

Transparency International ranks Iraq in 12th place in its list of the world's most corrupt countries.

- 'Investigation must produce results' -

Video footage from cameras at the picnic site, posted online, showed a severely overcrowded vessel beginning to list as water comes aboard.

As it capsizes completely, it traps dozens of people under its hull.

Authorities had warned people to be cautious after several days of heavy rains led to water being released through the Mosul dam, causing the river to rise.

Videos showed a fast-flowing, bloated river and dozens of people floating in the water or trying to swim around the partly submerged boat.

Abdel Mahdi announced Friday that a crisis team was being set up, comprised of top provincial officials but not the governor of Nineveh, of which Mosul is the main city.

The team would have "executive powers... to carry out an investigation as quickly as possible, punish those responsible and decide on compensation", the prime minister said.

He and President Barham Saleh have visited the site of the accident as the search continued for bodies, some of which were carried far downstream by the strong current.

While war and jihadist attacks have claimed hundreds of thousands of lives in Iraq in recent years, such accidents are relatively rare.

But corruption and the dismal state of public services in Iraq were triggers for widespread protests last year.

IS turned Mosul into its de facto capital after sweeping across much of the country's north in 2014.

The city spent three years under the group's iron-fisted rule until it was recaptured by Iraqi troops backed by a US-led coalition in 2017.

Survivors of Thursday's disaster were treated in hospitals heavily damaged by the months-long military campaign against the jihadists.

A mortuary was receiving bodies wrapped in white shrouds, many bearing the names of women.

A forensics official said many had yet to be identified.

Iraq's justice ministry said it had ordered the arrest of nine ferry company officials and banned the owners of the vessel and the picnic site from leaving the country.

Officials implicated in various scandals have fled Iraq in recent years.