Late peer Lord Janner allegedly began sexually abusing children as far back as 1955, a public inquiry has heard.
The peer died in December aged 87, soon after he was found unfit to stand trial over abuse claims, owing to dementia.
The Independent Inquiry into Child Sexual Abuse was told Lord Janner abused children over a 33-year period, arranging for those "in whom he had a sexual interest" to be brought to the Houses of Parliament.
His family denies the allegations.
The inquiry, chaired by Dame Lowell Goddard, will be the biggest in British history. It is set to last for five years and has a budget of £17.9m for this financial year alone.
Lord Janner's case is one of 13 areas it will initially focus on.
Speaking at the first preliminary hearing, counsel to the inquiry Ben Emmerson QC said: "The allegations in summary are that Greville Janner exploited children and perpetrated a full range of sexual offences against them, including what would now be termed in English law as rape.
"In relation to a number of the complainants it is alleged that Janner abused his position as an MP by arranging for children in whom he had a sexual interest to be brought to the Houses of Parliament."
The offences are alleged to have taken place in children's homes and hotels between 1955 and 1988 - previously the oldest allegations to emerge against him dated back to the late 1960s.
Wednesday's preliminary hearing was told that 17 complainants in the Janner case had been given core participant status in the inquiry, meaning they have the right to make statements, see documents and seek permission to ask questions.
The inquiry has not received an application for core participant status from Lord Janner's family.
The hearing also heard submissions about the anonymity of some witnesses, and whether public hearings, due to begin in September, will be televised.
Director of Public Prosecutions Alison Saunders originally decided the peer should not be charged when the allegations emerged because he was suffering from dementia, but this was overturned by an independent review.
A trial of the facts was due to take place at the Old Bailey, but was dropped when Lord Janner died.
It would have heard evidence from alleged victims, but decided only whether the ex-MP committed the physical acts of abuse - and would not have made a finding of guilt or a conviction.
'Findings of fact'
Referring to the Goddard inquiry as a whole, Mr Emmerson said: "The task is vast but critically important."
He continued: "Allegations of the involvement of politicians in child sexual abuse are reported on the one hand as evidence of a paedophile conspiracy... and on the other as evidence of a modern-day witch-hunt.
"It is the role of this inquiry to move from the realms of rumour and speculation, allegation and counter-allegation, to the assessment of objective facts."
He cited figures indicating that between April 2012 and March 2014 more than 50,000 children were identified as victims of sexual abuse, but said the true number may be as high as 450,000.
"These figures should answer the doubts of anyone who questions the need for this inquiry," the QC said.
As part of her wider inquiry, Dame Lowell, who served as a judge in her native New Zealand, will investigate the way in which public bodies handled child sex abuse claims.
The inquiry is unable to make findings of criminal or civil liability, but Mr Emmerson added: "No-one should be in any doubt that, where the evidence justifies it... the panel will make findings of fact on allegations of child sexual abuse."
He said it "will also need to recognise the damage that can be caused by false accusations of sexual abuse".
Who was Lord Janner?
- Born in Cardiff in 1928
- Served in the Army and studied at Cambridge before becoming a barrister and then QC
- Labour MP for Leicester North West and then Leicester West from 1970 until retiring in 1997, when he was made a life peer
- Diagnosed with Alzheimer's disease in 2009
- Suspended from the Labour Party in April 2015
- Ruled unfit to stand trial over allegations of child sexual abuse on 7 December 2015
- Died two weeks later, aged 87