Three of the men, who have all been convicted of robbery, had previously mounted successful escapes.
The prisoners were transferred on a private coach with a private driver because no prison vehicles were available.
The coach broke down two miles from Blundeston prison in Suffolk, from where 10 inmates were being transferred to jails in London. Six of the most violent men were allowed to continue their journey in a second coach accompanied by only five prison officers. Normally a dozen officers would guard six violent inmates.
During the wait for a replacement vehicle, one prisoner taught the others how to slip their handcuffs by dislocating their thumbs.
The officers, who did not know that some of the prisoners had previously been escapers, were overpowered and badly beaten.
The prisoners then seized their personal prison files which were being transferred with them on the coach. They ripped the papers up and threw them out of the windows. They then changed into their own civilian clothing which the prison authorities had allowed on to the coach as part of the prisoners' personal possessions.
After taking control of the coach on the M25, they ordered the driver to go to the Archway area of north London where they made their escapes.
Among those at large are Lee Mitty, serving 11 years for robbery, who absconded from Littlehey prison, Cambridgeshire, in March 1993 and remained at large for 18 months.
Warren Edwards, serving eight years for robbery, has made three previous successful prison escapes in 1991, 1992 and 1995.
Gary Staggs, who was sentenced to 10 years for robbery in 1993, had previously escaped from court cells.
Also on the run are Christopher Ward, serving 12 years, David Currey, nine years, and Stewart Warwick, nine years.
The six had formed a gang at the prison who were involved in a fight with a rival group of prisoners that resulted in one inmate receiving severe injuries including knife wounds to his face.
The Prison Service said: "There were tensions inside Blundeston that led to these prisoners being moved."
Harry Fletcher, assistant general secretary of the National Association of Probation Officers, said: "Public safety has clearly been compromised.
"This is another catastrophic blunder which should never have happened.
"There needs to be an inquiry into why a commercial bus was used to transfer six armed robbers, three of whom had previously escaped."
Bev Lord, vice-chairman of the Prison Officers' Association, said: "These prisoners had already demonstrated their propensity to violence and the lives of the prison officers were unreasonably put at risk."Reuse content