Story highlights Thousands of police are deployed in Belfast to prevent possible outbreaks of violence

The parade marks the centenary of the signing of the pro-union Ulster Covenant

Up to 30,000 people are expected to take part in the march

Tensions have been high this summer between Catholics and Protestants in the city

Thousands of people are taking part Saturday in a march through the Northern Ireland city of Belfast that many fear could inflame tensions between Catholics and Protestants.

Over the course of the day, up to 30,000 people were expected to join the Ulster Covenant parade, held to mark the 100th anniversary of the signing of a pro-union document which helped shape Northern Ireland's history.

The parade left from Belfast City Hall Saturday morning, headed for the grounds of Stormont, beside the Parliament Buildings.

Concerns over potential disorder along the route center have focused on an area near St. Patrick's Church , where violence flared up between Catholics and Protestants earlier this month.

The parade passed near the flashpoint Roman Catholic church, near the city center, without incident Saturday morning but must still return along that route in the evening.

Hundreds of police officers are stationed along the route in that area, with many more elsewhere in the city, as the Northern Ireland police service braces for possible trouble.

About 2,000 people took part in the morning parade feeding into the main demonstration, in which tens of thousands are expected to take part.

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The parade, a special event for the Ulster Covenant centenary, is expected to be one of the biggest held in years.

Hundreds of parades take place across Northern Ireland each year, the majority involving the Protestant Orange Order and associated organizations, although pro-Irish nationalists also have marches.

The Northern Ireland Parades Commission rules on which marches are allowed to take place and which are banned, in an effort to keep friction to a minimum.

Most parades pass off peacefully, but when members of one community march near or through neighborhoods dominated by another, violence sometimes occurs.

The rioting in Belfast earlier this month, which left dozens of police officers injured, was some of the worst seen by the city in years.

Tensions have been high all summer, and behind-the-scenes talks over Saturday's highly symbolic parade did not succeed in brokering a compromise between the Catholic and Protestant communities over the staging of the event.

The Ulster Covenant was a document signed in 1912 by nearly half a million people in opposition to steps by the government in Westminster to introduce "Home Rule," or self-government, in Ireland within the United Kingdom of Great Britain and Ireland.

The majority of the island gained independence in 1921, following two years of conflict. But six of the nine counties of the province of Ulster chose to stay in the United Kingdom, eventually becoming the country of Northern Ireland.

In the late 1960s, the conflict between mainly Protestant unionists, who want Northern Ireland to remain part of the United Kingdom, and largely Roman Catholic nationalists, who want it to be reunited with the rest of Ireland, exploded into a political and sectarian war, known as the Troubles.

The three decades of ensuing violence between the Irish Republican Army and loyalists claimed the lives of more than 3,000 people, most of them north of the border. While the Good Friday Agreement of 1998 effectively ended the conflict, suspicions between Catholics and Protestants remain.

Under the terms of the landmark accord, terrorist groups on both sides dumped their weapons, and Sinn Fein, the political affiliate of the IRA, now work with pro-British politicians in Northern Ireland's power-sharing government.