In Belfast, the main parade left City Hall at 12:00 GMT and made its way to Custom House Square where there will be a free concert.
In Downpatrick the celebrations began on Tuesday, but on Saturday the Celebration of St Patrick begins at the St Patrick Centre.
A concert in St Patrick's Square began at 13:00 GMT.
In Londonderry, the Voyage of Discovery themed parade also began at 13:00 GMT.
More than 900 community and voluntary groups are taking part.
In the Republic of Ireland, the largest parade will be held in Dublin.
Archbishop of Dublin Diarmuid Martin said St Patrick's Day was a time for Irish men and women around the world to "celebrate Irishness" and "celebrate our identity".