Trap doors in floorboards and liquor bottles on backseats -- Iraq's intelligence service on Monday detailed how the Islamic State (IS) group's car bomb cell eluded the police for months.

The suspected leader of the group was arrested riding a bicycle in an upmarket neighbourhood, a spokesman told AFP at the intelligence headquarters in Baghdad.

The Iraqi national intelligence service (INIS) announced on Sunday it had arrested 31 people responsible for planning and carrying out 52 attacks in Baghdad in 2014 and 2015.

"When we first picked up a scent in our hunt for this network, we organised surveillance that lasted six months," spokesman Fahim al-Atraqchi told AFP.

"Then we set up a task force and arrested all of them in 72 hours," he said. "We arrested the emir (leader) as he was riding his bicycle in Mansour."

Intelligence officials have released footage of the 31 suspects, lined up in brown jumpsuits at a detention facility, with their bomb-making equipment and weapons in front of them.

The intelligence service would not provide any names and a senior official, who would not give his name, said investigators were still looking for 10 cell members.

"The reason for the decrease in attacks in Baghdad over the past three weeks is the arrest of this network," Fahim al-Atraqchi told AFP.

But it took time to pin down the car bomb network, whose members had become very astute at dodging the countless police checkpoints that gridlocked Baghdad.

- 'Ingenious' -

The investigation revealed that members of the cell would get through checkpoints by ostensibly displaying bottles of alcoholic beverages on the back seat, Atraqchi said.

"They were trying to fool the police into thinking they were drinkers and dispel any suspicion they could be religious extremists," he said.

The intelligence service seized 10 vehicles in the course of the raid, all of which had explosives concealed in them, either to carry out an attack or to avoid capture by committing suicide.

One senior officer showed AFP false compartments on some of the vehicles, as well as the dozens of counterfeit number plates that were seized in the raid.

One of the items displayed by INIS was a modified Koran box, in which explosives were found.

Some cars had a trap door cut out in the floorboard so that bombs could be dropped in an area without having to get out of the vehicle and attract attention.

"They were very ingenious," the officer said.

According to Atraqchi, the dismantled cell was responsible for deadly bombings in the Shiite neighbourhood of Sadr City, one of the most frequently targets in Baghdad.

The suspects, who were carrying forged ID cards for the mostly Shiite volunteer Popular Mobilisation units, are also suspected over bombings in the busy central district of Karrada and many other parts of the capital.

"We were looking for targets that would grab headlines," one blindfolded suspect told AFP after being authorised to speak by an intelligence officer.

The father of four said he became radicalised during the four years he spent at Camp Bucca prison in southern Iraq.

He did not say why he was arrested but Camp Bucca, the formerly US-run facility where IS supremo Abu Bakr al-Baghdadi was also detained, is believed to have acted as an incubator for jihadists.

"They (other inmates) convinced me and others that we were being unjustly treated and should defend ourselves," the suspect, aged around 40, said.

"I had been working in a mobile phone shop and my life was good, but when I joined them, I stopped thinking for myself. I feel regret now," he said.

Some attacks have been carried out in recent weeks in Baghdad, which at one point last year used to be rocked by car bombs almost every day, but their number has dropped this year.

The years-old nightly curfew on Baghdad was lifted last month.