The websites of the Brazilian government and President have fallen victim to hacker group Lulz Security.
In a Twitter posting, LulzSec said "Tango Down" and linked to the two sites.
Both are thought to have been taken offline by distributed denial-of-service attacks (DDoS).
LulzSec recently signalled its intent to target the systems of governments and associated organisations around the world.
In the past month, it has attacked the UK's Serious Organised Crime Agency, the US Senate, Sony, and the broadcaster PBS, as well as a number of games companies.
DDoS attacks are regarded more as malicious activity than hacking, because the sites' computer systems are not broken into.
Instead, they are deliberately overloading with traffic, such as communication requests or so-called 'e-mail bombs'.
The group tweeted "Our Brazilian unit is making progress. Well done @LulzSecBrazil, brothers!" shortly after the two sites went down.
LulzSec also commented on the case of 19-year-old Ryan Cleary who was arrested in Essex on 21 June on suspicion of hacking websites.
The group said that he was not part of their organisation.
Metropolitan Police Commissioner, Sir Paul Stephenson, described Mr Cleary's arrest as "very significant".
Although he declined to discuss the details of the case, the Commissioner added: "The challenges around cyber-crime are extremely significant and deeply worrying across the gamut of cyber-crime from personal information to how a paedophile can enter a child's bedroom."