The new legal sports betting market in the U.S. just experienced its busiest four-day stretch ever as the NFL season kicked off.

GeoComply, a company that provides geolocation services to sportsbooks, tracked 58.2 million online transactions that took place Thursday through Sunday with sportsbooks in 18 states and the District of Columbia. It marked a 126% increase from the same period at the start of the 2020 NFL season.

New Jersey had the highest volume of sportsbook transactions, followed by Pennsylvania, Michigan, Arizona and Illinois, according to the GeoComply data released Monday. Arizona launched its betting market Thursday, just hours before the Dallas Cowboys and Tampa Bay Buccaneers opened the 2021 NFL season. More than 271,000 newly created sportsbook accounts in Arizona combined for more than 6.1 million transactions during the four days, according to GeoComply.

"The data tells a remarkable story about the growth of the industry in a short period of time," Lindsay Slader, managing director of gaming for GeoComply, said in a release announcing the data.

Licensed bookmakers are now operating in 26 states and Washington, with more jurisdictions expected to come online in the coming months and years.

"It's probably going to be one of our biggest weekends ever," DraftKings sportsbook director Johnny Avello told ESPN on Sunday night. "All the numbers look very positive."

Online sportsbook PointsBet also reported record betting interest in both the number of bets and amount wagered on the NFL's opening Sunday.

The expansion of legal sports betting in the U.S. hasn't hurt Nevada bookmakers, though. John Murray, executive director of the SuperBook at Westgate Las Vegas, told ESPN that the betting handle this past weekend was up approximately 20% compared to the same weekend last year.

"It was great to hear the crowds in here again on a full NFL Sunday," Murray said. "The mobile betting dominates the landscape these days, but we loved seeing so much over-the-counter business back at the SuperBook after it fell off in 2020 due to the pandemic and occupancy restrictions."

Jeff Stoneback, director of BetMGM sportsbooks in Nevada, said the betting handle on college football also is growing, adding that his books took 50% more action on the Saturday before Labor Day than they did during the same weekend in 2019.

"It's just growing and growing and growing. It's amazing," Stoneback said. "It's all across the country now, and there's constant commercials for betting. Everyone's so much more exposed to it."

Overall, sportsbooks reported a winning Sunday, as NFL underdogs went 11-4 against the spread with eight outright upsets.

"I couldn't have asked for a better day to start the season," Tom Gable, sportsbook director for The Borgata in Atlantic City, New Jersey, said.