One of the largest proposed mixed-use development projects in Boston’s history is now underway.

Demolition began last month on a 486-unit residential tower that marks the first phase of Bulfinch Crossing, National Real Estate Advisors and HYM Investment Group’s 2.9 million-square-foot mixed-use redevelopment of the Government Center Garage at 1 Congress St. that was approved last January by the Boston Redevelopment Authority.

Vertical constructi