As word of the attacks spread internet users rushed to the web for updates, with traffic surging to a peak at midday.
The BBC's news site managed to stay up and running, but suffered delays throughout the day. In an effort to speed up delivery, the site stripped many images from its web pages - a tactic employed by websites during the attacks on New York on September 11, 2001.
Traffic figures for www.bbc.co.uk are not expected until later today, but the corporation is already predicting the highest level of traffic in its history.
Sky's news website had 1.7 million visitors during the day - the equivalent of a month's traffic during normal times.
Also inundated was the website for news agency Reuters, which suffered a technical fault after experiencing five times the normal rate of traffic yesterday. The fault was fixed by the afternoon.