Baidu claims to have around 70 percent of China's Internet search market, and has only been down once before, for half an hour three years ago.

After Tuesday's incident, the company issued an apology to its users via e-mail. In its message, the company accuses hackers for attacking the website for their own personal interests. It says the incident has warned them of a new phenomenon of hackers attacking domain name servers, or DNS, instead of Baidu's own servers.

The firm is calling on DNS companies to resolve technical problems in their security. The case is still being investigated.