Tens of thousands of people and organisations were participating in a protest against the NSA’s mass surveillance on Tuesday, bombarding members of Congress with phone calls and emails and holding demonstrations across the globe.

Dubbed “The day we fight back”, the action saw scores of websites, including Reddit, BoingBoing and Mozilla host a widget inviting users to pressure elected officials.

The online demonstration saw more than 18,000 calls placed and 50,000 emails sent to US congressmen and women by midday Tuesday. Physical protests were planned in 15 countries.

“The goal of the day we fight back is to stop mass surveillance by intelligence agencies like the National Security Agency,” said Rainey Reitman, activism director at the non-profit Electronic Frontier Foundation, which helped organise the events.

“This is a unique political moment in the fight for surveillance reform. The leaks of 2013 shed light on surveillance abuses really unlike anything we had seen before that.

“Really it kickstarted an international debate about privacy rights which led to major shifts in public opinion polls as well as international pushes for surveillance reform.”

The EFF website Photograph: /EFF.org Photograph: EFF.org

NSA contractor Edward Snowden leaked documents detailing the scale of government surveillance to the Guardian in 2013. A series of stories published in 2013 showed how the US government was collecting metadata from Americans’ telephone and email communications.

Participating websites carried a highly visible banner on Tuesday bearing the motif “Today we fight back.” The banner encouraged readers to enter their phone numbers and email addresses to contact members of Congress and urge them to back the Support USA Freedom Act, which would end the bulk collection of Americans’ records, and oppose the Fisa Improvements Act, which seeks to legalise and extend the NSA’s surveillance programs.

Reitman told the Guardian that over 100,000 people had signed an international petition opposing mass surveillance within a few hours of its launch.

Electronic Frontier Foundation, which fights for online free speech and privacy, organised the day of protest with civil liberty campaigners Demand Progress and a coalition of prominent organisations and websites including Reddit, Greenpeace, ACLU, Tumblr and Amnesty International.

Anonymous showed support for the action, with a lengthy statement protesting “this police state nightmare”, as did, perhaps less obviously, Google, which emailed members of its “Take Action” web freedom group encouraging them to take part.

Google, Yahoo and Facebook revealed the extent of the data they had been forced to hand over to US government authorities earlier this month. The disclosures showed that between them the internet giants had disclosed details pertaining to tens of thousands of accounts.

Google, Yahoo, Facebook, Microsoft and others are behind the Reform Government Surveillance push which calls for governments around the world to have their ability to monitor users’ information limited.