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Photo: The Canadian Press. All rights reserved.

A police service of Northern Ireland officer watches Orange order members passing St Patricks Roman Catholic church in Belfast city centre, Northern Ireland, Saturday, Sept. 29, 2012.(AP Photo/Peter Morrison)

A large Protestant march through the heart of Northern Ireland's capital has begun peacefully amid a heavy police presence.

Tens of thousands are expected to participate in one of Belfast's biggest parades in years, and authorities have deployed in force to prevent street clashes with Northern Ireland's Catholic minority.

The march in Belfast commemorates one of the most significant dates in unionist history.

The six mile march from Central Belfast to Stormont marks the 100th anniversary of the Ulster Covenant, to oppose Home Rule for Ireland in 1912. 

The Unionist marches are a long standing irritant between Northern Ireland's two main religious communities and show no sign of fading away, despite a successful peace process.

Nearly every aspect of the marches, from the parade route to the music played, is fought over by both sides.