No one has claimed responsibility for the distributed denial-of-service or DDoS attack, RT.com reported Monday.
The technical preparations required for such an attack suggests it was the work of professionals or an organized, sophisticated group, RT.com spokeswoman Anna Belkina said.
Several RT websites have been the targets of DDoS attack in the past, she said.
In August 2012 a hacker group calling itself AntiLeaks, saying it opposed Julian Assange's WikiLeaks project, claimed responsibility for an attack.
Sunday's attack was the largest of its kind so far experienced by RT.com, Belkina said.