Groups including the American Civil Liberties Union, Amnesty International, the Electronic Frontier Foundation and Yahoo-owned Tumblr were on the roster of backers of an online protest called "The Day We Fight Back".
By mid-morning here, more than 20 000 telephone calls had been made to members of the US Congress to oppose privacy "intrusion" by the National Security Agency, according to Demand Progress executive director Segal.
"Winning this fight is going to require a broad coalition of individuals, organisations, and businesses, one that is coalescing around today's activism and will persist until we win the fight against mass suspicion-less surveillance," said Segal, whose internet freedom organisation claims 1.5 million members.
A thedaywefightback.org website provided simple tools to call or e-mail US legislators, and even provided a script of what to say.
Participating websites were to provide links, banners or other tactics to get visitors to barrage members of Congress with Twitter messages, Facebook posts, emails, or calls opposing US online spying.
Revelations about US surveillance practices at home and abroad have sparked an overdue debate about US surveillance laws and programmes, Google vice president of public policy Susan Molinari said in a blog post.
"Google recognises the very real threats that the US and other countries face, but we strongly believe that government surveillance programmes should operate under a legal framework that is rule-bound, narrowly tailored, transparent, and subject to oversight," Molinari said.
Protest backers hoped to harness the collective power of internet users to advance a USA Freedom Act that would reform surveillance practices.
"By overwhelming every member of Congress's phone line, inbox, and Facebook and Twitter accounts with our demand to end government mass surveillance, we can deliver a message impossible to ignore," ACLU legislative counsel Michelle Richardson said in a released statement.
"The internet helped kill Sopa and Pipa, now it can roll back the surveillance state by helping to pass the USA Freedom Act."