"The streets are not packed with people but they are having celebrations later in the day, as well," the CBC's Margaret Evans reported from Belfast.
Protestant organizers were expecting an estimated 30,000 people to take part, she said.
The focus of the march is the 100th anniversary of the Ulster Covenant, a petition that led to the partitioning of Ireland and the eventual formation of Northern Ireland.
Unionist marches have been a longstanding irritant between Northern Ireland's Protestant and Roman Catholic religious communities, and have continued 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.
"The police presence is enormous," Evans said.
The marchers are following a lengthy parade route to the parliament building, Evans said. One part of the city given particular police scrutiny was the area around St. Patrick's cathedral, in North Belfast.
The parade route goes right by the church and it's where clashes broke out last month," Evans said.