BEIJING—Baidu Inc., China's top search engine, said hackers disabled its Web site early Tuesday, and some Internet users reported seeing signs that an Iranian group was behind the attack.

Users in China reported seeing a banner for the "Iranian Cyber Army," complete with an Iranian flag and a shattered Star of David, when they tried to access Baidu's home page Tuesday.

Screen shot of the Baidu Web site on Tuesday morning

Marten Strassburg, a Swedish citizen living in Beijing, said he saw the site defaced with images of the Iranian group around 10:30 a.m. local time (9:30 p.m. Monday EST). Mr. Strassburg and dozens of others posted screen shots of Baidu's defaced site online.

For most of Tuesday morning, Baidu remained unavailable in China, with users seeing just an error message when they attempted to visit the site.

Last month, a group also calling itself the Iranian Cyber Army attacked Twitter, temporarily disrupting access to the U.S.-based social-networking site. The Iranian Cyber Army also appeared to have attacked an Iranian reformist Web site.

It was unclear why the group, which appears to sympathize with the Iranian government rather than anti-Tehran protesters, would attack Baidu.

Farsi text that ran across the top of the page shed little light on the matter, saying, in translation, "The Cyber Army of Iran was created to oppose the meddling of foreign and Zionist Web sites in Iran's internal affairs and to prevent these Web sites from spreading false news and lies aimed at creating divisions."

Twitter has become a tool for Iranian dissidents to communicate and organize, and Chinese nationals began expressing sympathy with Iranian protesters last month through Twitter. Though Twitter is blocked in China, these Internet users have found ways around the limits through the use of proxy servers outside of China.

But the Internet free-speech advocates on Twitter aren't fans of Baidu, which is seen to be in good standing with Beijing. Foreign Web sites such as Google Inc. have been periodically blocked by the Chinese government for linking to pornographic or politically sensitive material, but Baidu hasn't had similar problems with Chinese censors.

"Services on Baidu's main Web site www.baidu.com were interrupted today due to external manipulation of its DNS (Domain Name Server) in the U.S.," the company said in a prepared statement. "Baidu has been resolving this issue and the majority of services have been restored."

Baidu declined to comment on any possible Iranian involvement in the incident.

China's main state broadcaster, CCTV, reported the outage on its noon news broadcast, and suggested that users visit Baidu's Chinese domain name, www.baidu.com.cn, instead.

By the afternoon, both Baidu sites were accessible, with a few users reporting some periodic disruptions.

However, some Web sites with Iran's ".ir" domain names, including one linked to Iran's state-owned broadcasting network, appeared to have been hacked by Chinese programmers. The home page of one such site, www.diabetes.ir, contained a warning in fragmented English against intrusion on Chinese Web sites. It was signed "We form China."