Allegations that British troops carried out "terrifying acts of brutality" against Iraqi civilians have been made in the High Court.
They were made by lawyers representing 192 Iraqis asking for a public inquiry into British detention practices between 2003 and 2008.
The court will decide whether alleged mistreatment was "systemic".
It will also consider whether an inquiry set up by the defence ministry is independent enough to investigate.
The hearing before two judges is expected to last three days.
As well as unlawful killings, there are claims of beatings, hooding, sleep deprivation and sexual humiliation including being made to watch pornography.
Women, the elderly and children were among the victims, according to an 82-page document presented in court.
Lawyers said they were still collecting allegations of abuse almost a decade after the invasion of Iraq, and had hundreds of further claims in the pipeline.
The Ministry of Defence has paid out more than £15m to settle more than 200 claims of mistreatment and unlawful detention, with many more cases being negotiated.
But it argues that a wide-ranging public inquiry would be disproportionate and premature.
Lawyers for the Iraqis say the Iraq Historic Allegations Team (IHAT), created by the UK government, is investigating only individual cases. But they say the abuse was systemic and only a fully independent inquiry will satisfy the UK's international human rights obligations.
Michael Fordham QC, appearing for the Iraqis, told the High Court "enough is enough".
He said it was necessary "for somebody independent to grasp this nettle and pursue the truth of what happened, and its implications, compatibly with the rule of law. That is the big point".
Lt Col Nicholas Mercer, the legal adviser during the invasion in 2003, said he believed there was more than just a rogue element responsible for abuse.
"I reported abuse of prisoners nine days into the war, on 29 March of that year. And I left Iraq in July of 2003 and a number of cases emerged and the mantra at the time was this was a few bad apples.
"However... this has grown and grown and grown. And indeed last year there were 162 cases settled by the Ministry of Defence at the cost of £8.2m, so it no longer looks like just a few bad apples."
Two judges sitting in London, Sir John Thomas (President of the Queen's Bench Division) and Mr Justice Silber, are hearing the accusations.
Ministry of Defence lawyers are opposing the application.
MoD submissions made to the court, written by Philip Havers QC and James Eadie QC, stated that if evidence of systemic abuse of civilians did emerge, consideration would be given to holding "a broad-ranging public inquiry" along the lines being asked for by the Iraqi applicants.
The MoD lawyers also said: "Comprehensive steps have been taken by the secretary of state, and will continue to be taken, to ensure lessons are learned from events in Iraq.
"In particular... a significant amount of work has already been done to ensure that MoD policies and training on tactical questioning and interrogation are lawful and fit for purpose."
Outside of court, a ministry spokeswoman said: "The establishment now of a wide-ranging public inquiry to consider alleged systemic issues would be premature and disproportionate.
"It is also likely that any single public inquiry would take much longer than the investigations already being undertaken."
It is the second legal challenge by law firm Public Interest Lawyers, which is representing the Iraqis. One of its lawyers said there were a further 871 cases that should be considered.
The Court of Appeal ruled last year that IHAT did not have "the requisite independence" and said ministers had failed to comply with obligations under human rights legislation.
Following that ruling, ministers removed the Royal Military Police from IHAT and replaced them with Royal Navy Police officers.
But the Iraqis say that does not go far enough and that a fully independent, judge-led inquiry remains necessary.