China is struggling to restore several government websites that international hacking group Anonymous says it attacked in a protest against Chinese internet restrictions.

On a Twitter account established in late March, Anonymous China listed the websites it says it hacked over the past several days. They include government bureaus in several Chinese cities, including in Chengdu, a provincial capital in southwest China.

Anonymous activists have defaced websites around the world. They are engaged in political causes, including opposition to the global clampdown on file-sharing sites and defence of the secret-spilling site WikiLeaks.

Some websites that Anonymous said it attacked were working, and government officials denied the sites were ever hacked.

In a message left on one of the hacked Chinese sites - cdcbd.gov.cn, a home page for Chengdu's business district - the hackers expressed anger with the Chinese government for restrictions placed on the internet.