By Ivan Little

BELFAST (Reuters) - Police in Northern Ireland mounted their biggest security operation in 20 years on Saturday when 30,000 Protestants marked the centenary of one of the most historic events in a province scarred by decades of sectarian violence.

Eight Protestant Unionist organizations, including members of the Orange Order, marched through Belfast to celebrate the signing of the Ulster Covenant by half a million of their ancestors, a pact opposing the introduction of devolved government in Ireland.

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

Most parades across the province 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 on duty outside a Catholic church in Belfast as 2,000 Protestants and bandsmen passed peacefully by, although they are due to return past the potential flash point again after 1800 local time (1700 GMT).

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 Provisional IRA which now shares power in the local assembly with their former Unionist foes, said they were unhappy with the behavior of a number of bands.

Marchers went on to gather at Belfast's City Hall, where the covenant was signed, and then marched 10 kilometers (six miles) to Stormont, the home of Northern Ireland's power-sharing government where religious services, music and dancing were held.

(Editing by Padraic Halpin and Sophie Hares)