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On Wednesday, as many sites around the Web participated in virtual protests against two Congressional antipiracy bills, some opponents to the legislation took their demonstrations offline and into the real world.

The New York Tech Meetup, an eight-year-old trade organization that has nearly 20,000 members, called for those who oppose the proposed bills to rally in Midtown Manhattan outside the offices of Senators Charles E. Schumer and Kirsten E. Gillibrand, who co-sponsored some of the proposed legislation.

Close to a thousand protesters gathered in metal pens that the police had set up on the sidewalk.

“A lot of people in Washington are scratching their heads today because conventional wisdom is that SOPA would have passed by now,” said one speaker, Timothy Karr, strategy director for an organization called Free Press, referring to one of the bills. “But conventional wisdom is wrong, and it is no longer business as usual in Washington.”

Several local start-ups, including CrowdTap and LocalResponse, said earlier that they were planning to shut down their offices and bring all their employees to the event.

Sebastian Delmont, 38, who works at StreetEasy, a real estate site, said that about half of his co-workers had come to the protest. He said the risk to his company from the legislation was low, “but our worry is that they are building something like a Great Firewall, like in China and the Middle East.”

Sarah Hromack, who manages the Web site of the Whitney Museum, said she was at the protest as someone who cared about visual content online. Ms. Hromack said she was worried about the long-term implications of the legislation. She said she had participated in some online shows of support for the movement, “but getting out of our offices is also important.”

Similar rallies are planned for San Francisco and Seattle.

The offline events mirror online efforts, which include the darkening of several major Web sites, including English-language Wikipedia (although it is still possible to access the encyclopedia’s content through several clever workarounds), Reddit, Boing Boing and the comedy video site My Damn Channel . Tumblr, a blogging platform, is also giving its users a tool to let them black out what information they see when they log into their accounts in protest.

Several sites have not gone dark, but have blanketed their pages with information about one of the bills, the Stop Online Piracy Act, known as SOPA; these include Google, Craigslist and I Can Haz Cheezburger, a hub for humorous pictures of cats.

Google, whose SOPA page calls for visitors to sign a petition to Congress, said on Wednesday afternoon that 4.5 million people had done so.

The mobile restaurant finder UrbanSpoon is restricting access for its users, and some news organizations, including Wired.com are protesting the legislation by blacking out content on their Web sites.

Flickr, the photo site owned by Yahoo, gave users a tool that let them black out their photos “to deprive the Web of the rich content that makes it thrive.”

Such an outpouring of support around a political cause is atypical in the tech world, which tends to limit its gatherings to technology demonstrations, social events and launch parties around new products and services.

But Jessica Lawrence, the managing director of the New York Tech Meetup, said this legislation — which its most vocal opponents say could lead to censorship and thwart the innovation of technology start-ups — is sparking an unusually vigorous response.

“The tech community has not typically been engaged in political issues but that is changing, especially for smaller start-ups and companies that aren’t as big as Facebook and Google that have someone on staff for legislative issues,” she said. “These small companies would be left in the dark without anyone to represent their ideas.”

Mark Zuckerberg, chief executive of Facebook, weighed in against the legislation on his own Facebook page Wednesday afternoon, saying: “The world today needs political leaders who are pro-Internet. We have been working with many of these folks for months on better alternatives to these current proposals.” Within an hour, more than 200,000 Facebook users had clicked the “Like” button on his post. Facebook did not make any site-wide changes to support the protests.

While there were blackouts on the Web, a whiteout prevented critics of the proposed legislation from congregating in Seattle. An overnight snowstorm dumped several inches of snow on the city, scuttling plans for a protest. “If our goal is to educate people, it will be pretty hard to find people to educate today,” the organizers of the rally said in an update announcing the postponement of the rally.

The Seattle area is home to technology companies like Amazon and Microsoft that have voiced opposition to SOPA. Amazon did not black out its Web site, instead providing a link from its home page to Net Coalition, a group opposing the legislation.

Nick Wingfield contributed reporting.