Thousands of spectators lined the streets of South Boston on Sunday for the city’s annual St. Patrick’s Day parade, one of the largest celebrations of Irish heritage in the country. Marching bands, veterans’ groups, local organizations and floats made their way along the traditional route through the neighborhood. City officials coordinated security, traffic restrictions and public transit plans for the large crowds gathered for the event. The parade is a major annual tradition in Boston, a city with deep Irish roots, and often coincides with celebrations of Evacuation Day, which commemorates the departure of British troops from Boston in 1776. Evacuation Day this year is on Tuesday.