If you had been on the Strand in London on the day that the high court was considering how to proceed with scores of civil actions against the News of the World for its phone-hacking escapades, you would have seen a peculiar sight. About 30 people were gathered on the steps of the court, the palms of their hands painted red, bearing banners that read: "Murdoch's men caught red-handed."

On the same day, outside a Sainsbury's store in Godalming, Surrey, where the culture secretary, Jeremy Hunt, was holding his weekly constituency surgery, another group of 25 people had gathered. They were leafleting shoppers about the News of the World scandal and calling on the government to delay approval of Rupert Murdoch's bid to takeover BSkyB until a full public inquiry could be held.

Both events were the work of one of the most successful of a new breed of internet campaigner, in this case a global activism network called Avaaz, which means voice in Urdu and several other languages. It put out an alert to its half a million UK members calling for activists to attend the two stunts, with impressive results.

To get a sense of what Avaaz is and how it operates you have to switch the lens 3,000 miles to a pleasantly light-filled office with great views overlooking Union Square in Manhattan. This is where Avaaz has its headquarters – if an organic network of internet activists can be said to have a headquarters.

Avaaz, formed in 2007, has more than eight million members in 193 countries and can claim to be the largest online activist community in the world. This year alone it has attracted an extra one million members and it is now wholly self-funding with about $20m (£12m) raised so far in online donations.

"We have no ideology per se," says director Ricken Patel. "Our mission is to close the gap between the world we have and the world most people everywhere want. Idealists of the world unite!"

A Canadian who holds dual British citizenship, Patel was involved in student activism while at Oxford University studying PPE and later at Harvard. After three years working for aid groups around the world and a stint at the UN, he witnessed the power of the internet as a volunteer for the US liberal campaign MoveOn.org.

What MoveOn tries to do with domestic American politics, Avaaz applies globally. Its weekly meeting of staff, held via a Skype conference call, gives a taste of its ambitions. With the Guardian listening in, several of the 35 Avaaz staffers join the call from their bases in San Francisco, Toronto, Mexico City, Rio, London, Paris, New Delhi and Sydney.

The team cheered when the US staff began by talking about this week's news that Bradley Manning, the alleged WikiLeaks source, had been moved to a new, more lenient, prison. Avaaz had organised an online petition signed by 530,000 members calling on President Barack Obama to "end the torture" of the US soldier.

Next, the Canadian staff talked about an Avaaz campaign to force the Ottawa government to release a report into alleged misuse of G8 funds, while the Delhi staff gave an update on the health of Anna Hazare, an activist they are backing who has been on hunger strike in protest at Indian political corruption.

While most of Avaaz's projects are initiated by the staff themselves, every few days they survey a random collection of 10,000 members to ask them which campaigns they want to prioritise.

They also monitor constantly online statistics that reveal which campaigns are attracting most interest among members, enabling the membership itself to chose the network's focus. "Democratic accountability is hard-wired into the way we work. Each campaign is only as successful as the number of people who choose to join it," Patel says.

The idea of a campaign against the News of the World was received with great enthusiasm by Avaaz's members, particularly in the UK.

Patel says: "We have long seen Rupert Murdoch as a powerful threat to the health of our democracies through his domination of the media environment. When we polled our members, they were strongly in favour of trying to stop his takeover of BSkyB until a full public inquiry into News Corp could be held."

Avaaz teamed up with its fellow online lobbying group 38 Degrees to send 60,000 submissions opposing the Murdoch bid to Ofcom.

Late last year, Avaaz members sent 50,000 messages to David Cameron and Hunt calling for a review by the Competition Commission, supported by a petition signed by 400,000 global members. To press home the point, it targeted 10 key constituencies of politicians involved in the bid decision and invested in TV and newspaper adverts arguing against the takeover.

The overall aim, according to Avaaz's Bristol-based campaign director Alex Wilks, was to get "tens of thousands of citizens signing up, even just for five minutes, so they can express themselves and make a difference".

But he says it is not enough just to sit back and rely on the internet to do the heavy lifting: "We have also to get into politicians' faces and make sure they know how many people feel passionately about what they are, or are not, doing."

• This article was amended on 26 April 2011. In the original, Canada's capital was spelled Ottowa. This has been corrected.