A construction site in Bethesda causes major headaches for residents, customers and businesses. The fence built to shield construction shows how close the site is to family homes. (Astrid Riecken/For The Washington Post)

First came the warning sirens. Then the blasts, unleashing seismic jolts across downtown Bethesda.

Nearby buildings shook. A neighbor thought a meteor hit. The saleswoman in the tea store thought a car hit her building. Others thought it was an earthquake.

Meanwhile, as the blasts continued, the guy behind the counter at the Dunkin’ Donuts has been a calming presence.

Don’t worry, he has told startled customers. It’s just the construction blasts.

Since the beginning of De­cember, construction crews have been setting off thunderous ­explosives as they blast through layers of bedrock to make way for a $150 million mixed-use construction project in the center of Bethesda Row.

The blasts occurred regularly in the first two weeks of last month, then stopped during the holidays. They are expected to return this month.

When completed, the downtown Bethesda development will bring 88 condominiums, 162 apartments and 40,000 square feet of commercial space, transforming what was once a parking lot across from the Barnes & Noble store. It will also bring 940 public parking spots to an area starving for parking space.

The project, by PN Hoffman and Stonebridge Associates, began in April and is expected to be completed in two years. It will have two structures: one nine stories, the other five. There will be almost 60 units of moderately priced and workforce housing, said Doug Firstenberg, a developer with Stonebridge who is overseeing the project. The developers had expected to break ground in 2007, but they started this year because of the economic downturn.

But the interim excavation work has been causing headaches for business owners and shoppers. About a year ago, two surface parking lots providing nearly 300 metered spaces were closed so that utility work could begin. This caused the already frustrating hunt for parking in downtown Bethesda to become even more troublesome.

About a month ago, construction workers encountered the bedrock, a mix of schist, gneiss and quartzite. The first blast, on a chilly morning in early December, took some by surprise.

Neighbors rushed out of their homes, wondering whether it was an earthquake.

“Will my house stay up at the end of this?” asked one neighbor, Asimina Coroneos, only half-joking, on a recent Friday afternoon.

Coroneos was checking e-mails in her bedroom when she heard the boom and felt the shaking. She rushed downstairs and left her house without stopping to grab a jacket.

“I thought something huge like a meteor hit my house,” she said.

Karen Cirrito, a manager from Amethyst, a jewelry store on Bethesda Row, felt the vibrations and saw some of the store’s metal tree sculptures move.

Construction workers drill holes into the bedrock, place the explosives into the holes, and cover them with dirt and heavy rubber mats. As the explosion goes off, the surrounding rocks fragment and cause nearby concrete and dirt to ripple.

Cirrito said now that she’s grown accustomed to the tremors, they are no longer a bother. The larger problem, she said, is the reduced parking. People are avoiding shopping in downtown Bethesda, she said. Compared with last year, sales have dropped by half in her store.

“We’re all doing poorly now,” she said.

Nonetheless, neighbors and store owners say the developers are listening to their concerns. Over the past two weeks, there has been a reprieve. But soon the sirens will start blaring again. In the meantime, neighbors worry about the effects of construction on their homes, many of which were built in the 1930s. Many have started noticing cracks in their walls and back yards.

The developers took video and photos of the houses before construction began. After the construction ends in 2015, they will take more photos and determine what damage, if any, the work caused. The developers told residents they will then compensate them for damage caused by construction, neighbors said.

Firstenberg said they are studying the first blasts to see how they can further “minimize the impacts.”