AT&T had been pushing hard to win approval of the deal since it was first announced in March, arguing it would create jobs and expand broadband access.
The merger won the support of dozens of Republican and Democratic lawmakers, as well as the backing of major unions, who pointed to the fact that AT&T employees are unionized, while T-Mobile’s are not.
But the public-interest groups argue the deal would kill competition in the wireless market, leading to higher prices for consumers and fewer jobs.
Andy Schwartzman, policy director for Media Access Project, a nonprofit law firm, said he was not surprised by the Justice Department's decision to block the deal. "The law could not be more straight forward," he said.
But he added he was surprised the Justice Department chose to act when it did, rather than waiting several more months.
The public interest groups said that the Justice Department decision puts pressure on the Federal Communications Commission to act quickly in its own review of the merger.
In a statement following the Justice Department's announcement, FCC Chairman Julius Genachowski said his agency's review was not complete, but echoed the Justice Department's concerns that the deal would harm competition.
Unions, conservative organizations and some wireless trade groups were disappointed by the decision.
“Today's announcement by the Department of Justice is surprising and unfortunate for American consumers, our rapidly growing wireless sector, and the promise of mobile innovation in the United States," said Jonathan Spalter, chairman of Mobile Future, a group that represents communication companies and organizations.
Steven Law, president of Crossroads Grassroots Policy Strategies, a conservative advocacy group, said the decision was part of the Obama administration's "nonstop war of harassment against American businesses that has frozen job creation in this country."
--This story was updated on Sept. 1 at 2:37 p.m.