The investigation could result in an apology from the Pope in the year 2000.

BBC Rome correspondent Orla Guerin: Will the Vatican apologise?

But although the inquisition is regarded as one of the darkest chapters in the Church's history the Vatican conference will hear evidence that it wasn't that bad.

It is part of an introspective examination in the run up to the new millennium as the Catholic Church prepares to apologise for its past sins.

Death was a mercy

The inquisition was created to deal with people who contradicted Roman Catholic teachings.

It is impossible to estimate how many were persecuted, but no-one denies that torture was used and some victims were burnt alive.

It was considered an act of mercy to strangle a victim before setting him alight.

At face value it might seem that all of this merits an unequivocal apology but some historians argue that, by the standards of the time, the inquisition was actually quite progressive.

For example, the accused could be represented by a lawyer and hearsay evidence was not enough to seal someone's fate.

They also claim that contrary to popular belief, only about two per cent of those who came before the inquisition were actually put to death.

Critics of this revisionist interpretation say the percentages are immaterial. They want the Vatican to make a comprehensive apology for what was done in the name of religion.

Bishops and cardinals also hounded

Pope Gregory IX instituted the papal inquisition for the apprehension and trial of heretics in the 13th century.

Inquisitors did not wait for complaints, but sought out people accused of heresy, including witches, deviants and blasphemers.

The Roman Inquisition was established in 1542 by Pope Paul III folowing growing alarm at the spread of Protestantism.

When Pope Paul IV stepped up the pursuit of suspects the following decade he did not even spare his own bishops and cardinals.

One of the Roman Inquisition's most famous victims was the astronomer Galileo Galilei.