The micro-blogging service was heavily used by all those involved with the meeting, be they in the debating chamber, quizzing politicians after briefings, or protesting beyond the police cordon.
Social media was in use prior to the event too. The Metropolitan Police analysed activity on websites such as Facebook, MySpace and many others to see how many were likely to join the call for protests at the G20.
Plans for how many policemen and women should be put on the streets during the G20 were drawn up with regard to this analysis.
Bridging fences
But it was during the two-day event that social media, in particular Twitter, came in to their own. Thousands of messages marked with a #G20 tag passed through Twitter and organisations such as IndyMedia had their own tags that helped to collate information about what happened during the protests.
Did they organise that via Twitter? I'd be surprised - it's a very public place to talk about something you don't want the police to hear.
—Rory Cellan-Jones, Technology correspondent, BBC News
For Anjali Kwatra, a spokesperson for Action Aid, Twitter was essential in giving supporters outside a glimpse of what was happening inside the Excel centre.
"It definitely helps make that link with what people are out on the streets protesting about," she said.
It also let supporters know about the questions Action Aid campaigners were putting to politicians and their efforts to get their message across to the leaders of G20 nations.
The immediacy of the medium also appealed, said Ms Kwatra, adding that the 140 character limit gave messages an informality over other ways of communicating.
"We are also blogging, but that takes time and goes on to the Action Aid website, where people have to come in and look at it," said Ms Kwatra.
"It's a mix of both the observational and informative," she said.
Ground force
Dominic Casciani, from BBC News, who has been mixing with protesters across London, said many of the groups used Twitter as a way to reach out to supporters more quickly than they could with e-mail or text messages.
For instance, he said, the Climate Change campaigners who set up the camp on Bishopsgate used Twitter to rally supports to carry supplies - namely baked potatoes and beans - to the site where they planned to set up tents.
"It's not the defining tool because it's never going to replace the mobile phone or text message to close friends," he said, "but there's clearly something in broadcasting to subsets of people that are looking for specific information."
Richard Roaf, a spokesperson for People and Planet, said Twitter had been invaluable in helping smooth the organisation of the Bishopsgate Climate Camp.
Twitter was used to send regular updates as the camp was being set up and to let people know how it was progressing, he said.
With net and computer access literally thin on the ground, Twitter was, he said, the best option for reaching everyone.
And, he added, it proved even more valuable when the police moved in to break up the camp and scatter the protesters.
"Once the situation changed with the police, then it let people back here know what was going on," he told the BBC. "It was like a live TV channel, rather than an update after it had all happened."
For many, Twitter was a way to find out about and get to the independent media reports from people caught up in the protests. One of the messages most regularly re-tweeted was a link to video footage of the police breaking up the Climate Camp.
Using Twitter as an unofficial noticeboard let those who had lost touch on the ground find out what had happened to friends and fellow protesters during and after the police clear-up, said Mr Roaf.
Friends concerned about what had happened to people they knew were reassured to see messages from them posted on Twitter reporting that they were fine.
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