To accommodate a growing number of early-morning swimmers at Walden Pond in Concord, state park officials will open up the main parking lot at 5 a.m. on a trial basis for the rest of the summer.

Until recently, the lot did not open until 7 a.m., which meant some of the swimmers would illegally park along Route 126 or in the small gift shop lot that could not accommodate all of the cars.

Walden, a 102-foot-deep glacial kettle-hole pond made famous by Henry David Thoreau, has become an increasingly popular spot for cross-pond swimming, especially among triathletes.

S.J. Port, a spokeswoman for the state Department of Conservation and Recreation, said the change appears to have solved the parking problem along the state highway.

“DCR made the decision to try opening Walden’s main visitor lot earlier after being alerted to a burgeoning public safety issue along Route 126,’’ she said. “Early-morning visitors needed parking, DCR had a parking lot; it seemed a good fit.’’

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Port said officials will monitor the new hours and review them in September. The opening time may be adjusted as the sun rises later in the day, she said.

Swimmers welcome having a legal place to park.

“As things are now, it’s perfect,’’ said Juan-Andres Leon of Somerville. “People are very happy about it. It’s a matter of seeing what happens after Labor Day, and next year.’’

Port said visitors, regardless of the time of day, must pay $5 a day to park, or display a seasonal parking pass, which is $35. Staff is on hand at the Walden Pond State Reservation starting at 5 a.m. to open the bathhouse and visitors center, and operate the parking gate.

Port said state officials have been surprised by the number of early-morning visitors to the park. On July 18, the first day the state opened the lot at 5 a.m., 52 cars arrived between 5 and 7 a.m. Port said about half of the cars had at least two people in them.

The busiest time was between 5:30 and 6 a.m., when 20 cars arrived.

Also between 5 and 7 a.m., 12 people arrived on bikes, four ran in, and 10 walked in.

“From what I hear, many visitors are enthusiastic about the change,’’ Port said.

Now that the main lot is open, Port said, the gift shop lot, which previously was available to early visitors, is blocked off so all spaces are available for employees.

Thoreau wrote extensively about the beauty of the pond while living in a cabin nearby from July 1845 to September 1847. His book, “Walden,’’ is credited with helping to inspire awareness and respect for the natural environment. Walden Pond has been designated a National Historic Landmark and is considered the birthplace of the conservation movement.