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The tent is up. The stage is ready. Late Wednesday afternoon, city officials and developers will celebrate the official start of work on a building that has been years in the making.

The project on the former site of the Winthrop Square Garage will have its groundbreaking ceremony, with Boston Mayor Martin J. Walsh joining Millennium Partners founding partner Chris Jeffries to make speeches and toss ceremonial dirt into the air.

Work actually began last month on the $1.35 billion tower after Millennium closed on its purchase of the site and cut a $102 million check to the city for the property. The developer will also write another check, estimated at about $60 million, based on how many square feet of condos it sells.

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At 691 feet, the new tower will be the fourth-tallest in Boston — after 200 Clarendon (790 feet), the Prudential Tower (749 feet), and One Dalton (742 feet) — and the tallest structure in the Downtown Crossing area.

Excavators were moving dirt on Tuesday and getting ready to dig out the foundation of what will be a massive building squeezed onto a small lot in the heart of the Financial District.

When the building, known as Winthrop Center, is finished in 2022, it will hold about 1.6 million square feet of office space and condos, with a “great hall” for a lobby, open to the public and designed as a sort of indoor living room for downtown.

The tower proposal was the subject of a two-year debate over the shadows it would cast, as well as the design, before winning approval from the Boston Planning & Development Agency.

Proceeds from the sale of the city-owned site to Millennium have so far been dedicated to parks and public housing around the city. Representatives of the various neighborhoods where those projects are being funded — from Orient Heights in East Boston to Franklin Park in Dorchester — will also be on hand for Wednesday’s event.

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In its news release announcing the groundbreaking, Millennium emphasized that Wednesday’s event is for “all of Boston.”

After the ceremony, everyone will presumably go watch the Red Sox play in the second game of the World Series — like most all other Bostonians.


Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter @bytimlogan.