"Brazil is crap in these Olympics," read the main message inserted in several places on the committee's site, Globo.com and other news outfits said.
The website was hit twice in the early hours of Tuesday before the committee pulled it off-line.
Throughout the day it remained inaccessible, with the message: "This site is temporarily down for maintenance."
Brazil has so far won six medals in Beijing: a gold in swimming and five bronze (in swimming, judo and sailing).
In comparison, the country won a total 10 medals in the 2004 Olympics, 12 in the 2000 games and 15 medals in 1996.