The footage was as immediate as it was electrifying. Egypt's biggest demonstrations in three decades were relayed virtually live to the rest of the world by people posting mobile phone pictures via Twitter, the US-based microblogging service.
The protests were largely organised by apparently leaderless online activists on another hugely popular website, Facebook. They asked people to stand up, proclaiming: "We have to show the world that we are not a cowardly, submissive people."
The swift and unexpected overthrow of Zine El Abidine Ben Ali's 23-year-old regime in Tunisia and the mass unrest in Egypt have sparked a lively debate on how far social media networks can challenge governments around the world.
Internet activism, it seems, is no longer the hobby of a chattering class but an empowering motor that has mobilised thousands of protesters, first in Tunisia and now across Egypt.
Internet restrictions are not uncommon in the region, but in the past week they were toughened in Damascus and Tehran. Syrian authorities on Wednesday banned programs that allowed access to Facebook Chat from mobile phones. Iran last Sunday officially launched a cyber police unit to confront social networks that spread "espionage and riots".
The Egyptian government itself blocked access to Twitter on Tuesday evening. There also were reports on Wednesday that Facebook was being intermittently blocked, although the Egyptian authorities denied responsibility.
Egyptian activists fought back on Wednesday by disseminating technical advice to overcome these obstacles to enable the organisation of protests to continue. And a group of hackers known as "Anonymous", which attacked Tunisian government websites this month, yesterday warned in a Facebook posting of reprisals against Egyptian authorities if they block internet access for protestors.
Some sceptics scorn the term "Twitter revolution" to describe the tumultuous events in Tunisia and the protests in Egypt. They argue that social media did not play a decisive role in overthrowing Mr Ben Ali's government.
Political change, they say, still requires people willing to protest in the streets. Geeks sitting at their keyboards will not do it.
Moreover, some insist, regimes like Iran's have turned the tables by using the internet to track down and arrest dissidents. At the same time, governments are using social media networks to spread their propaganda and intimidate opponents.
Iran ruthlessly crushed huge pro-democracy protests that erupted in the wake of President Mahmoud Ahmadinejad's fiercely disputed re-election in June 2009. As well as word-of-mouth, demonstrators had used social media networks and mobile phones to mobilise protests and publicise their message.
A prominent sceptic of the liberating power of cyberspace is Evgeny Morozov, a Belarus-born scholar at the New American Foundation. He has just published The Net Delusion, a book which argues that hopes of fostering democracy through internet technology are unrealistically utopian - and can backfire.
"As we know from the post-protest crackdown in Iran, the internet has proved a very rich source of source of incriminating details about activists; the police scrutinised Facebook groups, tweets, and even e-mail groups very closely," he wrote in his blog last week.
Many Middle East experts counter that such arguments miss the point. Of course, they agree, the Tunisian upheaval was driven by several factors, including soaring prices, poverty, high unemployment, a bulging population and systems of rule that ignored the voices of ordinary people.
But the new social media acted as an indispensable accelerant, helping to organise protests while publicising the demonstrators' demands at home and abroad. Some compare the internet to the role played by the US-funded Radio Free Europe in helping to tear down the Berlin Wall.
"Facebook was massive in Tunisia. Not necessarily in organising the protests - a lot of which was done through word of mouth, and political, labour and student networks - but to actually disseminate news of what was happening," said Scott Lucas, a professor of American studies at Birmingham University in England.
"They were getting the news out on Facebook before Al Jazeera [which covered the protests closely] picked up on it," added Mr Lucas, who also runs Enduring America, a blog with Middle East coverage.
The Egyptian protests were in response to an online rallying cry spearheaded by the April 6 Movement, a group founded in 2008 by mainly well-educated young people. They launched a Facebook poll a few days before this week's demonstrations, asking: "Will you rally on January 25?"
Nearly 90,000 indicated they would, leading a few days later to one of the largest protests in Mr Mubarak's 30-year rule.
Gerald Butt, the Cyprus-based author of several books on the region, said the Tunisian revolt emboldened Egyptian demonstrators while social media ensured they turned out in big numbers. "There were calls for days of rage in the past but they fizzled out. The mood has definitely changed," he said in an interview.
Ethan Zuckerman, a senior researcher at Harvard University's Berkman Centre for Internet and Society, believes that social media - particularly Facebook - "likely played a significant role" in the Tunisian uprising.
Even before this month's historic events, tech-savvy Tunisian activists had stirred the pot by, for instance, releasing a provocative online video of their country's first lady on frequent shopping trips to Europe using the presidential jet.
"Any attempt to credit a massive political shift to a single factor - technological, economic or otherwise - is simply untrue," Mr Zuckerman wrote in Foreign Policy, the US-based magazine on global affairs.
"But … online media did play a role in helping Tunisians learn about the actions their fellow citizens were taking and in making the decision to mobilise."
