Twitter on Thursday is battling a denial-of-service attack which has shut down the micro-blogging website for several hours in the morning.
"The site is back up, but we are continuing to defend against and recover from this attack," Twitter announced in its latest update about the situation.
"On this otherwise happy Thursday morning, Twitter is the target of a denial of service attack," Twitter co-founder Biz Stone confirmed earlier in a blog posting.
"Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users," he said.
Denial-of-service attack can knock a website offline by massive junk requests from millions of computers controlled by the attackers.