Rivals in the violent underworld of Unionism

The two groups at each other's throats in Belfast have existed side by side in the city's loyalist districts throughout the Troubles, often on reasonably friendly terms though with periodic eruptions of feuding.

The two groups at each other's throats in Belfast have existed side by side in the city's loyalist districts throughout the Troubles, often on reasonably friendly terms though with periodic eruptions of feuding.

The UVF is the older of the two, dating back to about 1966. In that year, some three years before the Troubles really began, its members killed three people in Belfast.

The imprisonment of some of its leading members reduced its activities to a minimum for several years, but with the outbreak of violence in the late 1960s membership soared. The group was always seen as more secretive than the UFF, though in the 1970s its leaders, sporting black leather jackets, were not too difficult to spot.

One of its most notorious units was the so-called Shankill Butchers, a gang which carried out cut-throat murders of Catholics in the mid-1970s. It was headed by Lenny Murphy, later assassinated by the IRA, who was generally described as a psychopath. During the Troubles, the UVF killed about 550 people, the majority of them Catholic civilians. Although statistically the most violent of loyalist groups, it has more recently reduced its violence and supported the peace process.

Its father-figure is Augustus "Gusty" Spence, who was jailed for one of the 1966 murders and in later life urged the organisation to move away from violence. The UVF's political wing, the Progressive Unionist Party, is headed by two of his protégés, David Ervine and Billy Hutchinson, members of the Northern Ireland Assembly.

The UFF, also known as the Ulster Defence Association, killed approximately 400 people. In the early 1970s it initially emerged as a collection of vigilante groups, who combined to show their numerical strength at large rallies and marches.

Within a short time its more militant members turned to killings. Like the UVF, its assassins used guns and bombs, though in the early 1970s some of them were responsible for a number of killings which involved knives and torture.

One of those jailed for two knife killings, John White, has emerged from prison after more than a decade to become chairman of the UDA's political wing, the Ulster Democratic Party. The UFF's best-known faces are Johnny Adair who has been involved recently in protests, and Michael Stone.

Mr Stone, who killed three people at an IRA funeral in Belfast's Milltown cemetery in the late 1980s, now supports peace and is apparently a force for calm. But the UFF seems to be under the control of Mr Adair, a remarkably aggressive figure even for the aggressive loyalist underworld.