Torrential rain caused chaos across several parts of Iraq Thursday, with flood waters sweeping a toddler to her death and damaging camps for displaced people.

The storm that hit Baghdad Wednesday evening was unusually violent and the first after a long, dry summer.

According to a spokesman for the meteorological department, 54 millimetres (2.1 inches) of rain fell on the Baghdad region in 24 hours, causing thigh-high flooding on some streets.

A three-year-old girl died when she was swept away by the water at a camp for displaced people near Tuz Khurmatu, about 220 kilometres (135 miles) north of Baghdad, officials said.

Hospitals in Baghdad as well Najaf, to the south of the capital, reported hundreds of cases of patients checking in with respiratory ailments caused by humidity.

The poor condition of infrastructure in Baghdad, the Arab world's second-largest city with an estimated population of more than eight million, resulted in spectacular flooding.

The government declared Thursday a national holiday, knowing that many residents would be unable to reach work due to the effect on transportation.

Many motorists were unable to move their vehicles, stranding them on the roadside in the most flooded areas as others were marooned in the middle of deep pools of water.

In one video posted on social media, the staff in a Baghdad hospital were seen wading knee-deep in water.

Residents of some neighbourhoods of the capital spent most of the night bailing rain mixed with sewage water from the ground floor of their homes.

"Forget the furniture, I am afraid of the diseases this can spread. Shame on our past leaders, who allowed everything in this city to rot," said Ahmed, from northwestern Baghdad.

A wave of protests erupted this year in Iraq over the poor quality of services, including the lack of electricity when summer temperatures topped 50 degrees Celsius (120 Fahrenheit).

- 'Even worse' forecast -

In areas around the capital, rivers of mud wrecked the tented camps set up for people displaced by conflict in the western province of Anbar and other regions.

In Amriyat al-Fallujah, a town in Anbar just west of Baghdad, 500 tents were destroyed and around 1,000 families had to be moved to mosques and schools.

"The people were scared. They could see their tents swept away and disappearing in the mud," Shaker al-Issawi, the local chief, told AFP.

The governor of Anbar, Suhaib al-Rawi, asked the government to open a bridge that separates his province from Baghdad and which displaced people are usually prevented from crossing.

The United Nations said more than 10,000 people living in the camps had been affected.

"Preliminary assessments reveal that some 2,000 displaced families have been affected in various locations around Baghdad," said Natalia Micevic, a spokeswoman for the UN's refugee agency.

She said the UNHCR had rented tankers and pumps to drain the camps, was moving to replace the tents and distribute basic domestic items to the families in need.

More than 3.2 million people have been displaced by conflict in Iraq since the beginning of 2014.

One of the unexpected consequences of the downpour was the accidental detonation of what a senior officer in the Anbar operations command said were hundreds of bombs planted by the Islamic State group to defend the provincial capital of Ramadi.

The improvised explosive devices, mostly in the areas of Albu Dhiyab and Albu Farraj, "were planted by the Daesh (IS) terrorists to stop the progress of the security forces north of Ramadi," the officer said.

"Hundreds of them went off as a result of the rain and humidity," he said.

Heavy rain continued Thursday afternoon, more was expected on Friday and weather forecasts predicted fresh thunderstorms next week.

"Starting on Monday, it could be even worse than this and last for three days," a researcher from the meteorological service said.