A major parade that Northern Irish politicians and security forces feared could spark fighting between Catholics and Protestant passed peacefully on Saturday after police mounted their biggest security operation in 20 years.

Some 30,000 Protestants marked the centenary of one of the most historic events in a province scarred by decades of sectarian violence, the signing of a pact by half a million of their ancestors opposing the introduction of devolved government in Ireland.

Eight Protestant Unionist organisations, including members of the Orange Order, marched through Belfast to celebrate, twice passing a Catholic church where violence had recently flared up.

More than 1000 members of the British security forces were among 3600 people killed during 30 years of the "Troubles" in the province which has a minority Catholic population.

The violence between mainly Catholic nationalists and pro-British Protestants was largely ended by a 1998 peace deal, but sporadic violence has grown in recent years and a far smaller march earlier this month incited three nights of rioting.

Most parades across Northern Ireland pass peacefully each year but violence often breaks out when marchers cross or pass close to rival communities, particularly during the divisive summer marching season which is nearing its conclusion.

Hundreds of police officers were outside the Catholic church as 2000 Protestants and bandsmen passed peacefully by as their march began and again when they walked by on their return leg without incident.

The Parades Commission, the body that decides whether or not marches can take place, ordered bands accompanying the loyalist marches to play only hymns as they walked by the church.

It also restricted to 150 the number of Catholic protestors, who waved black flags and held banners urging respect for their church.

Sinn Fein, the former political wing of the Irish Republican Army (IRA) which now shares power in the local assembly with its former Unionist foes, said it was unhappy with the behaviour of a number of bands.

Marchers also gathered at Belfast's City Hall, where the Ulster Covenant was signed, and marched 10 kilometres to Stormont, the home of Northern Ireland's power-sharing government where religious services, music and dancing were held.

- Reuters