The families, who accused the Police Complaints Authority of "pro-police bias", will picket the event, called Deaths in Police Custody: Reducing the Risks.
The conference was called to address growing concerns that many of the 500 deaths that have occurred in police care or custody in the past 10 years could have been prevented.
But Myrna Simpson, the mother of Joy Gardner, who died while police officers were trying to deport her as an illegal immigrant in 1993, said the event would be incomplete without the views of the families.
She claimed families had been told that they would be too emotional to attend a professional conference. Mrs Simpson said: "It is emotional every time our loved one's birthday comes around. Why should this be emotional?"
Ms Gardner, who died after being bound and gagged with 13ft of sticking tape, is one of seven victims of deaths in custody represented by the United Families and Friends Campaign, which is organising the picket. Ken Fero, a campaign spokesman, said: "Without our voice the conference will be a one-sided talking shop of the same professionals who have failed to prevent police custody deaths in the past."
The key speakers at the event will include the Police Federation chairman, Fred Broughton; the South Wales Chief Constable, Tony Burden and Dr Michael Knight, chairman of the Police Surgeons' Association, as well as a hospital consultant and a journalist.
The event, being held at Church House in Westminster, central London, next month, is also being boycotted by Inquest, the independent organisation, which for the past 17 years has monitored deaths in custody and supported the families of victims.Reuse content