In the face of the coronavirus’ spread across Massachusetts, construction sites in the city of Boston are being shut down.

Mayor Marty Walsh announced the decision in a press conference in front of city hall Monday afternoon, calling COVID-19 “one of the greatest public health challenges our city has ever faced.”

“These decisions that we make are not easy, but they’re out of an abundance of caution. They’re about protecting the workers and stopping the spread of this virus,” he said.

Starting Tuesday, all construction sites will have to stop work and focus on securing materials and enacting security precautions, work that must be completed by March 23. Walsh, in particular, asked firms working on wood-frame buildings to keep a close eye on their sites for fire risks.

Walsh said he estimates the move will impact “tens of thousands” of workers.

“We’re in the middle of a boom right now. Today’s a difficult decision to make,” he said. “Construction is at the core of our economy, here.”

During the closure, construction sites will be permitted to have skeleton crews on hand to ensure security. Walsh urged construction companies to not fire workers during the site closures.

The decision followed Gov. Charlie Baker’s statewide ban on gatherings of over 25 people.

The financial and legal implications are likely to become clearer in the coming week. Shortages of labor and materials are frequently cited in force majeure clauses in real estate contracts, which can be used to extend payment deadlines between developers, contractors and vendors.

Even before Walsh’s announcement Monday afternoon, some real estate deals were being put into jeopardy because of delays related to legal and financial companies transitioning to work-from-home arrangements, said Tyler Ballew, an attorney at Sherin and Lodgen in Boston.

“In any of these transactions where you’re depending upon the flow of money, those chains of approvals can break down a little,” Ballew said. “Not everybody is in the same office, and things don’t get approved the same way.”

Less than a week ago, multifamily and commercial projects were proceeding with a business-as-usual approach, with Boston-based Davis Cos. hosting a topping-off ceremony Tuesday for its 100 Shawmut luxury condo project in South End.

The Massachusetts Building Trades Council issued a statement in support of the suspension.

“We will be working with our unions and with our contractor partners to make sure every worker is safe and secure, and we are confident that by working together as a community, we will get through this difficult time,” general agent Brian Doherty said.