China has condemned as a "smear" a New York Times report saying Premier Wen Jiabao's relatives have accumulated billions of dollars.
The article said Mr Wen's family members "have controlled assets worth at least $2.7bn (£1.7bn)".
A Foreign Ministry spokesman said the report had "ulterior motives".
Both the NYT's Chinese and English sites are being blocked inside China, as are references to the report on micro-blogging sites.
"Some reports smear China and have ulterior motives," Foreign Ministry spokesman Hong Lei said when asked about the story in a daily press briefing. On the blocking, he said the internet was managed "in accordance with laws".
In its report, the New York Times said Mr Wen's relatives' holdings included property, insurance and construction firms.
"Many relatives of Wen Jiabao, including his son, daughter, younger brother and brother-in-law, have become extraordinarily wealthy during his leadership," the newspaper wrote.
"In many cases, the names of the relatives have been hidden behind layers of partnerships and investment vehicles involving friends, work colleagues and business partners."
The family's investments reportedly spanned several sectors. The newspaper cited one holding as Ping An, an insurance company which it said had benefited from reforms enacted in 2004 by a state body over which Mr Wen had oversight.
It said that partnerships controlled by Mr Wen's relatives, along with their friends and colleagues, had bought into the firm before its IPO, or stock market flotation, in 2004, and held as much as $2.2bn in the company in 2007.
The newspaper said both the Chinese government and Mr Wen's relatives declined to comment on the investigation, which was based on corporate records from 1992-2012.
No holdings were found in Mr Wen's name, it said, nor was it possible "to determine from the documents whether he recused himself from any decisions that might have affected his relatives' holdings, or whether they received preferential treatment on investments".
China is sensitive about reports on its leaders, particularly when it comes to their wealth.
A growing wealth gap is causing public discontent, as are the frequent corruption scandals involving government officials.
When, in June 2012, a Bloomberg investigative report examined the finances of the relatives of president-in-waiting Xi Jinping, the company's website was blocked in China - even though the report said there was no indication of wrongdoing by him or his family.
Mr Wen has been the Chinese premier for almost 10 years. He is due to step down in a power transition that begins on 8 November.
He is seen as a popular figure with the common touch, and is portrayed in state media as a leader with great concern for the lives of ordinary people.
A spokeswoman for New York Times said she hoped that full access to the websites would be "restored shortly" in China.
The BBC has also been affected, with the BBC World News channel blocked when a correspondent was asked about the story during a report, and the BBC News website blocked later on Friday.
On China's Twitter-like weibo platforms, keywords such as Wen Jiabao and the New York Times are blocked. Mr Wen's name, like most other Chinese leaders, has always been a screened keyword.
Some netizens did manage to post the article despite heavy and rapid censorship. A Sina Weibo user tweeted about the article from Kawagoe city in Japan, but his post was removed after 11 minutes.
"The Twist Your Waist Times says the best actor has $2.7bn of assets. I just wonder how will he spend it?" asked a Tencent Weibo user registered in the British West Indies territory of the Turks and Caicos Islands.
"Twist your waist" in Chinese characters sounds like New York when spoken, while "best actor" refers to Mr Wen, who critics say only pretends to be a people-first leader.