LAWRENCE — The doctors are in the building. Finally.

After a year and a half of sometimes tense negotiations and six months of frenzied construction, Pentucket Medical Associates employees spent Memorial Day weekend moving their offices from Route 114 in North Andover to the Riverwalk at 500 Merrimack St.

The move appeared to go smoothly, as doctors, nurses, technicians, phone operators and administrators pitched in to move hundreds of crates and boxes filled with supplies, computer equipment, files and more into the recently renovated space adjacent to Merrimack Valley Federal Credit Union.

It was a colossal task made even more difficult as the move, originally intended to be finished by May 31, was pushed up a week so that the walk-in clinic at the new center could open for patients over the weekend while the rest of the doctors' offices would be open today.

Linda Persichetti, site manager for the project, was still smiling late Friday afternoon as an army of construction workers put the finishing touches on the building while Pentucket medical staff began settling in and admiring their new surroundings.

"I'm smiling, we're in a new building," Persichetti said, noting that as the North Andover offices closed Thursday at 5 p.m., the Lawrence office was open for business Friday morning — at least partly.

She said six operators were answering phones in the basement of the three-story complex, housed in what was once the steam plant that generated electricity for the adjacent textile mills. If a patient needed assistance, they were transferred to a nurse. If emergency help was needed, they were transferred by ambulance to the Pentucket Medical Center in Haverhill.

"We've answered a lot of calls," she said. By Saturday morning, the new walk-in clinic was open. Today, the offices housing another 20 doctors and nurse practitioners were to open.

During a recent tour, Pentucket Medical President Tom Fazio touted the layout of the new building. While one side will house the walk-in clinic, with its own entrance, the other side of the 35,000-square-foot space will house doctors' offices for more routine types of visits.

The bottom level of the three-level building includes just enough parking for doctors. Also on the lower level is the phone bank where operators answer calls from patients. In addition, the lower level will have a café to be run by Sal Lupoli, who owns the building as well as Salvatore's Restaurant and Sal's Pizza. There also is a conference room and the blood lab. On the second level, a greeter will meet patients as they enter the building and direct them to the appropriate part of the building.

There are doctors' offices throughout the first and second floors. Parking for other staff members and patients will be in the main lot at the center of the Riverwalk.

"This is very lovely and very exciting," said Dr. Jane Thompson, who practices internal medicine and has been with Pentucket for 11 years. "There's plenty of room, interesting colors and the architecture really supports what we do."

She said the exam rooms were well thought out, with convenient equipment stations. Her second-floor office was directly across the hallway from her exam room. And while she was still missing her desk late Friday, she had everything else she needed, including her computer with patient files.

Her office had tall windows overlooking the Merrimack River. "The view is great and the move is going very well, all things considered," she said. "This has been in the planning for several months, and people like Linda have done a great job putting it all together."

Dr. Brian Murphy, chief of radiology at Lawrence General Hospital, who also runs Merrimack Imaging, the subcontractor doing the X-ray work at Pentucket Medical, said the move allowed the medical facility to make a major upgrade to digital imaging.

"It's a big difference," he said, noting that the North Andover X-ray lab used film, the industry standard for many years but which has been eclipsed by computerized digital images of X-rays.

The X-ray lab will be connected to Lawrence General Hospital over a secure Internet connection so that doctors there can look at X-rays taken by technologists at Pentucket Medical.

Susan Milne, one of the technologists taking the X-rays at Riverwalk, said she also was happy about the move and happy to be learning the new technology that goes with digital imaging.

"The new system is totally computerized and totally different," she said. She noted that she also was happy to be in the new facility.

"It's a beautiful building," she said.

Lupoli, owner of the Riverwalk complex, said the Pentucket project was one of the more complicated ones he's been involved with since he started renovating the 2 million square feet of mill buildings five years ago.

"This is without question the hardest project I've ever worked on as far as the build-out," he said. "It was a very difficult process. We negotiated for a year and a half, signed the lease in January, and they had to be in the new space by May 31. It was an intense construction schedule that had to take place."

He said contractors in some cases were working 24 hours a day, seven days a week. He said crews worked through the weekend, "right up until (last) night to finish it by (this) morning."

Indeed, on Friday afternoon workers were still installing door knobs, cutting rugs in the entryway, hanging mirrors and taking on other construction tasks to ready the building for staff and patients.

In all, more than 100 employees will work at the new facility.

Pentucket Medical CEO John Sarro said the move was necessary because the company has outgrown the smaller North Andover location. He said that office had been receiving 30,000 patient visits a year, up 40 percent in recent months. Further, parking at the old location was difficult and traffic on Route 114 has grown so bad that getting in and out of the complex was difficult for patients as well as staff.

The arrival of Pentucket Medical at Riverwalk adds to what is becoming a cluster of medical-related businesses at the complex, including New England Neurological Associates, Home Health Foundation and Merrimack Valley Elder Services.

Pentucket Medical Associates is owned by Partners HealthCare.

PMA at a glance

r 200 employees

r 52 doctors

r 20 nurse practitioners

r Offices in Newburyport, Haverhill, Methuen, Lawrence, Georgetown

r 75,000 patients

r $50 million company

r Part of Partners HealthCare System, a $6 billion company with 6,000 doctors