Under the first warm sunny day in weeks, Quincy city officials and LBC Boston executives took sledge hammers to the 1500 Hancock St. block to officially begin its demolition and construction of a new, modern, commercial and residential property for the city’s downtown.

In remarks before a crowd of city officeholders, administrators, construction workers and LBC executives and friends, Quincy Mayor Thomas P. Koch said this portion of the downtown area had been in decline for decades. But no more.

“Combined with other projects we have going, the revitalization is happening,” Koch said in a statement. “It’s really happening.”

The mayor and three LBC executives donned construction helmets and goggles and, armed with gold-plated sledgehammers, took the first official demolishing blows to the buildings. Full-fledged demolition begins immediately after final permits are issued.

LBC officials estimate construction of the $40 million project will take two years and create 153,000 square feet of commercial and residential space. The mixed-use 7-story building includes retail, restaurants, residential and a 24-hour galleria. The galleria will connect Hancock Street and the main parking area with an all-weather space with seating, art and kiosks.