US Internet firms protested on Wednesday against an anti-piracy bill they say will encourage web censorship.
Wikipedia, the online encyclopedia, has shut its English-language version for twenty-four hours.
The Stop Online Piracy Act is backed the movie and music industries.
They argue the measures are necessary to stop copyright infringment, a view disputed by champions of Internet freedom.
Lance Ulanoff, editor-in-chief of social media site Mashable, said the laws would be “truly draconian measures.”
“They can do virtually anything they want if they happen to notice that a website has used copyrighted content. They can block the site. They can block it from the internet,” he said.
But the bill does not have the full support of the White House.
President Barack Obama will not sign the bill in its current form.
His spokesman, Jay Carney, said Obama “will not support legislation that reduces freedom of expression, increases cyber security risk or undermines the dynamic, innovative global internet.”
Facebook, Twitter and Google also oppose the changes but have declined to join the shutdown.
High-profile supporters of the bill include News Corporation chairman Rupert Murdoch, who last week branded Google a “piracy leader.”