Business and other users were unable to access the microblogging site for at least 90 minutes as Twitter scrambled to deal with the problem.
Twitter and other companies including Facebook companies were victims of "a single, massively co-ordinated attack," according to blog post by Twitter co-founder Biz Stone.
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Facebook encountered network issues related to an apparent DoS attack that resulted in degraded service for some users, the company said in a statement.
Both Twitter and Facebook emphasised that no user data was at risk as unknown attackers flooded the sites with requests to block legitimate users.
Twitter is working with other companies to investigate the attack.
Stone said Twitter had worked hard to achieve technical stability, but yesterday's globally distributed attack was a reminder there remains a lot of work ahead.
Tony Dyhouse, director of the Cyber Security Knowledge Transfer Network, said the threat of malicious attack needs to be considered when running a business which demands a continuous online presence.
"Cyber attacks can lead to a loss of revenue and a damaged reputation," he said.
Paul Vlissidis, director at NCC group's Secure Test, said DoS attacks were becoming commonplace.
"The fact that Twitter said that it was 'defending' the attack suggests that it was malicious. Popular websites have to factor DoS attacks into their security strategies, as it doesn't look like they're going away any time soon."
There is little these sites can do against a well organised denial of service (DoS) attack, said Tony Dyhouse.