The U.S. National Security Agency's website
was not accessible Friday for several hours.
An NSA spokesman said the site went down during a scheduled update, dismissing claims the site had endured a denial-of-service attack.
A denial-of-service attack is an attempt by outsiders to make a network unavailable to its intended users.
Hacker groups Tweeted jokes about the failure of the government website.
The NSA has been at the center of a global uproar over its vast electronic surveillance operations, revealed in a series of leaks from former intelligence contractor Edward Snowden.
A coalition of groups known as Stop Watching Us is staging a rally Saturday in Washington to protest the widespread surveillance revealed in the Snowden leaks.