The Guardian's reporting on the National Security Agency's surveillance activities has been recognised with two top US news industry awards.

At a ceremony in Atanta on Saturday night, the Online News Association, the leading body in the US for digital news organisations, awarded the Guardian the Gannett Foundation award for investigative journalism and the Gannett Foundation watchdog journalism award, for its work on the issue of NSA dragnet surveillance.

The Guardian's coverage of the story began in June with the revelation that the NSA had been collecting the telephone metadata of millions of Americans. Since then the Guardian, with documents leaked by the former NSA contractor Edward Snowden, has reported extensively on the scale and scope of the NSA's activities, leading to legislative moves in the US Congress to rein in the agency.

The Washington Post has also reported on the documents, and more recently the Guardian has been working in collaboration with the New York Times and ProPublica. The investigation has also included the operations of GCHQ, the British government's surveillance service. The three journalists who broke the story were Glenn Greenwald, Laura Poitras and Ewen MacAskill.

Joshua Hatch, chair of the Online Journalism Awards and senior editor for data and interactives at The Chronicle of Higher Education, said: "It was a story that couldn't be ignored and that's what watchdog is. When that story came out, people not only in the US but around the world stood up and took action.

"The reaction speaks for itself. And it was done effectively online. They got the most important story of the year and they told it well. This was a combination of most important and told well. That's a tough combination to beat."

The editor-in-chief of Guardian US, Janine Gibson, who accepted the awards at the ceremony in Atlanta, said: "We're very proud to accept these awards from the ONA and the Gannett Foundation. It means a great deal to have the endorsement of our peers in the US media and we are thankful for their support.

"These awards are significant, not just to our team who have worked relentlessly for months on this story, but because they recognise the importance of difficult watchdog journalism in the public interest."

The OJAs are awarded by the Online News Association, the world's largest association of digital journalists, working in partnership with the University of Miami's School of Communication. Other winners on Saturday night included the Boston Globe, which was honoured for its coverage of the Boston Marathon bombing and other stories, the Texas Tribune and the New York Times.