A falling tree knocked out power for most of a Connecticut town Monday, which NStar officials said demonstrated the value of removing trees near main power lines.

NStar’s removal policy, instituted in 2010, has provoked controversy in several communities west of Boston, where residents and elected officials decried the elimination of trees that have grown in their neighborhoods for generations.

In late July, Brookline halted NStar’s planned removal of two Norway maples and 20 red oaks, while Needham officials sent a letter asking the utility to not go forward with planned clear-cutting around high-voltage transmission lines.

But the utility’s president said such cutting is necessary to prevent outages such as the one that affected Greenwich, Conn., Monday.

“This incident emphasizes in the most vivid way possible the widespread effects a single tree can cause on a utility’s transmission rights-of-way,” Werner Schweiger, president of NStar, said in a statement. “It’s critical to electric service reliability that we not allow trees to continue to grow near high-voltage lines.”

Shawn McDonnell, deputy fire marshal for Greenwich, said in a phone interview that the outage affected more than 27,000 customers of Connecticut Light and Power, a sister company to NStar, or about 99 percent of the New York City suburb.

“The town was pretty much black,” McDonnell said. “It put us into kind of a holding pattern for the day.”

The outage closed Greenwich’s downtown shopping district, McDonnell said, and left thousands of residents without air conditioning as temperature rose into the mid-80s.

While workers cut down the tree and cleared the power lines, they had to shut off power for about two hours to nearby rail lines serving Amtrak and the Metro-North Commuter Railroad, McDonnell said. But since the shutdown came at midday, McDonnell said, morning commuters were spared and evening commuters were unlikely to be affected.

McDonnell said the power went out around 10 a.m. Monday after a 75- to 80-foot ash tree began to lean on a main electrical transmission line on Summit Road in the town’s Riverside section.

The tree may have become unstable due to ground soaked by heavy rains that began late Sunday night and continued into Monday morning, McDonnell said.

An NStar spokesman said falling trees are the most common cause of outages and that the company has an obligation to address them so the utility can provide reliable service.

Michael Durand, the spokesman, said the utility understands concerns about tree removal and works with residents and elected officials to explain its process and address those concerns.

“Every day our arborists balance the aesthetic value of trees with our commitment to reliability,” Durand said. “One tree coming in contact with a transmission line can interrupt power to tens of thousands of customers, and we have an obligation and a duty to do everything we can to prevent that from happening.”

One of the customers affected was Bill Kulhanek, who has owned Elm Street Oyster House in Greenwich for 24 years. Kulhanek said the outage had cost him about $2,000 in lunch business that will be impossible to recover. “Oh it’s gone,” he said. “You just kiss it goodbye and hope it doesn’t happen too much.”

Kulhanek said he felt fortunate, though, that no food was damaged before power returned to the restaurant around 2 p.m. And Kulhanek said he was expecting a good turnout for dinner. “The phone’s been ringing off the hook because most of the town is still without power,” he said.