People turned to the net in their millions to check friends and family were fine following the London attacks.
News sites were strained as people sought the latest news from London
Some news sites got a month's traffic on Thursday and the huge surge in visitors caused problems for others.
Net monitoring services reported brief outages at some news sites and some pages took longer than normal to load.
E-mail traffic reportedly doubled in response to congestion on mobile networks which made it hard to get through to people in central London.
Keynote Systems, which monitors traffic on net backbones and to top sites, said that visitors to news sites surged for a five-hour period on Thursday morning as details of the attacks in London emerged.
It said that the download times for webpages on some of the UK's main news sites were eight times longer than normal.
In some cases pages took about 18 seconds to download and a quarter of requests did not get through.
"Users who were trying to access the information were seeing higher than normal delays, and at the same time some people weren't able to get through to some sites," said Roopak Patel, senior net analyst for Keynote.
Monitoring firm Netcraft said that the Reuters.com news site suffered brief outages on Thursday.
It said that the Reuters site responded quickly but often all it was showing visitors was an error message.
Reuters later said this was due to a technical problem and was unrelated to the surge in visitors.
In a bid to lighten the load on its servers the BBC reduced the number of images on the main www.bbc.co.uk webpage.
Despite this Netcraft said the site was intermittently available and took longer than normal to respond.
The BBC said its home page, bbc.co.uk, was available at all times during Thursday.
Some servers were restarted, meaning some people might have not been able to connect to that particular server for a minute or two.
The BBC News website (news.bbc.co.uk) generally coped with the greater than usual number of visitors.
Contingency plans to cope with significant events involve using the cacheing network of technology firm Akamai.
The Reuters news site briefly returned error messages
Colin Nesbit, head of operations at the BBC News website, said that usually images were handed over to Akamai to reduce the work that the BBC's servers had to do.
Akamai has a huge number of servers dotted around the net that hold copies of popular webpages.
But Mr Nesbit said that on Thursday almost all the BBC News website pages were handled by Akamai.
"They basically provided our network for most of yesterday," he said.
At peak time the BBC News website was serving up more than 1.7 gigabits of data every second, with 40,000 page requests per second. It is likely to be the busiest day in the BBC News website's history.
By late afternoon on Thursday Sky News said it had handled 1.7 million unique visitors. "That's the equivalent of a month's traffic on the site," said a Sky spokeswoman.
Netcraft also reported that there were brief outages around the times of the attacks on the Barclays and BAA websites.
MSN said that traffic to its site had doubled and that twice the usual number of people were using streamed video to keep up with events.
Many turned to e-mail to get through to people they could not reach via phone. The sheer number of mobile calls being made in central London meant that networks struggled to cope.
Mail filtering firm MessageLabs said that e-mail traffic doubled and it was seeing more than one million messages per hour on Thursday morning pass across its servers.
"We don't know what the traffic is, but we're guessing that it's 'Are you OK?' and 'Have you seen the news?' messages. But that's based on the e-mails we've been getting," said Alex Shipp, senior antivirus technologist for MessageLabs.