A cyber-attack has crippled a US-based website that has reported extensively on China's biggest political turmoil in years.

Boxun.com was forced to move to a another hosting service on Friday after its previous host said the attacks were threatening its entire business, said the website's manager, Watson Meng. He added that he believed the attacks were ordered by China's security services, but that it was unclear where they were launched from.

The assaults on Boxun's server followed days of reporting on Bo Xilai, formerly one of China's most powerful politicians, who was sacked as head of the Chongqing municipality and suspended from the party's politburo amid accusations that his wife was involved in the death of British businessman Neil Heywood.

The scandal has deeply embarrassed party leaders. Six years ago, when Shanghai powerful party chief Chen Liangyu was sacked in a corruption purge, Chinese social media was in its infancy and months went by with no word on the case against him.

The Bo scandal began to emerge in February when his former right-hand man and Chongqing police chief, Wang Lijun, visited the US consulate in a neighbouring city in an apparent attempt to seek asylum. Rumour abound on the internet of a spat with Bo, but neither the Chinese nor the US authorities revealed any details of the visit.

At the time, Bo admitted to not properly managing his staff, but it appeared he would keep his job and remain a candidate for the party's standing committee when a new generation of leaders is picked later this year.

But then suggestions began surfacing online that Wang was spreading the word about the alleged involvement of Bo's wife, Gu Kailai, in the death of Heywood, a business consultant with close ties to the Chongqing party chief's family. Those suspicions first appeared in a brief posting in early March by a reporter from the Southern Weekend newspaper group, who said he had received the information via a text message on 15 February from a number used only by Wang.

That happened after Chinese authorities took the police chief into custody on 7 February, so it was not known who sent the message. However, it was widely circulated online, and the foreign media flocked to Chongqing to investigate, making it had for the government to ignore the case.

A few weeks later, on 15 March, Bo was sacked, and on 10 April the authorities announced he was under investigation and that his wife and a household aide were suspects in the Heywood murder.

Boxun, which has reported on the scandal since early February, was brought down for several hours on Friday in a denial of service attack in which hackers deluge a website to paralyse it.

"We publish articles critical of the Chinese government so we're accused of having ulterior motives," Meng said. "But in the west, most media is critical of its government, so why can't we be?"

Foreign governments and companies often complain of cyber-attacks from China, although proving their origins and who the culprits are is rarely possible. Beijing denies that it uses hackers to attack web sites or steal secrets online.

Meng set up Boxun in 2000 to promote the pro-democracy movement, human rights, and expose corruption. Much of its material is submitted by readers. It has been the target of cyber-attacks before and has gone without advertising since 2005. The US government-funded National Endowment for Democracy provided funding for several years, but Meng says it is now wholly independent.

Not all of Boxun's reports have held water, but many of those alleging Gu's involvement in the Heywood death and Bo's falling-out with Wang have since been proven true or been corroborated by other sources.

Traffic to the site has increased 155% over the past three months, according to internet monitoring firm Alexa, with the second largest chunk of visitors coming from China, despite government blocks.

China heavily censors the internet and blocks Twitter, Facebook, YouTube and scores of other overseas sites. Government monitors swiftly remove sensitive postings and have tried to rein in the Chinese microblogging site Weibo by requiring proof of identification for new accounts and sometimes disabling sections where comments can be posted.

Still, the sites have a profound effect. Witness reports on a horrific train collision last year prompted disgust at officials' callousness and a sweeping safety review.

One reason why the crackdown has not been harder is because elements within the establishment also use it to attack rivals, spread misinformation or advance their own agendas, said Xiao Qiang, director of the China Internet Project at the University of California, Berkeley.

But they cannot completely control the online discussions or filter out all unwanted revelations, Xiao said. "Those facts and opinions generate pressure or create the conditions for the government to take actions such as firing Bo Xilai."