Three activist groups flew a blimp bearing the words "Illegal Spying Below" over the US National Security Agency's data centre in the state of Utah in protest against the federal government's mass surveillance programmes.
The 41-metre blimp flew over the hi-tech facility for an hour.
It was launched by the environmental group Greenpeace, which owns it, digital rights activists the Electronic Frontier Foundation and a conservative political organisation, the Tenth Amendment Centre.
An NSA spokesman declined to comment, but noted there was no restricted airspace over the data centre, housed in the grounds of a Utah National Guard camp.
The blimp protest coincided with the launch of an online campaign that rates members of Congress on actions the activists say either further or stop data collection efforts by the NSA.
On Friday, the agency for the first time released data on the scope of some of its most sensitive foreign intelligence-gathering efforts, saying it had targeted nearly 90,000 foreign persons or organisations for surveillance through US companies last year.
The release of the "transparency report" by the Office of the Director of National Intelligence, follows an order a year ago from US President Barack Obama to make public as much information as possible about certain sensitive surveillance programmes.
Some privacy advocates said the figure was not as high as they had expected, but that the report left out important data. It did not include figures on the number of Americans whose phone calls or e-mails were collected accidentally or because they were in contact with foreign targets.
"The intelligence community is hiding the extent to which this surveillance conducted without a warrant is impacting people in the United States, who have constitutional rights," said Gregory Nojeim, senior counsel for the Centre for Democracy and Technology.
Additional reporting by The Washington Post