SAN FRANCISCO — It started five years ago after a young engineer in San Francisco sketched out a quirky little Web tool for telling your friends what you were up to. It became a bullhorn for millions of people worldwide, especially vital in nations that tend to muzzle their own people.

But this week, in a sort of coming-of-age moment, Twitter announced that upon request, it would block certain messages in countries where they were deemed illegal. The move immediately prompted outcry, argument and even calls for a boycott from some users.

Twitter in turn sought to explain that this was the best way to comply with the laws of different countries. And the whole episode, swiftly amplified worldwide through Twitter itself, offered a telling glimpse into what happens when a scrappy Internet start-up tries to become a multinational business.

“Thank you for the #censorship, #twitter, with love from the governments of #Syria, #Bahrain, #Iran, #Turkey, #China, #Saudi and friends,” wrote Björn Nilsson, a user in Sweden.

Bianca Jagger asked, almost existentially, “How are we going to boycott #TWITTER?”

Zeynep Tufekci, an assistant professor at the University of North Carolina at Chapel Hill, took the other side. “I’m defending Twitter’s policy because it is the one I hope others adopt: transparent, minimally compliant w/ law, user-empowering,” she wrote.

Twitter, like other Internet companies, has always had to remove content that is illegal in one country or another, whether it is a copyright violation, child pornography or something else. What is different about Twitter’s announcement is that it plans to redact messages only in those countries where they are illegal, and only if the authorities there make a valid request.

So if someone posts a message that insults the monarchy of Thailand, which is punishable by a jail term, it will be blocked and unavailable to Twitter users in that country, but still visible elsewhere. What is more, Twitter users in Thailand will be put on notice that something was removed: A gray box will show up in its place, with a clear note: “Tweet withheld,” it will read. “This tweet from @username has been withheld in: Thailand.”

Think of it as the digital equivalent of a newspaper responding to old-fashioned government censorship with a blank front page.

“We have always had the obligation to remove illegal content. This is a way to keep it up in places where we can,” said Alex Macgillivray, general counsel at Twitter. “We have been working on this awhile. We needed to figure out how to deal with this as a company.”

The majority of Twitter’s 100 million users are overseas and it has several offices abroad working to expand its business and drum up local advertising. Twitter’s president, Jack Dorsey, said this week that it would open an office in Germany, which prohibits Nazi material online and offline.

The announcement signals the choice that a service like Twitter has to make about its own existence: Should it be more of a free-speech tool that can be used in defiance of governments, as happened during the Arab Spring protests, or a commercial venture that necessarily must obey the laws of the lands where it seeks to attract customers and eventually make money?

Tim Wu, a professor at Columbia Law School and author of “The Master Switch,” said the changes could undermine the usefulness of Twitter in authoritarian countries.

“I don’t fault them for wanting to run a normal business,” he said. “It does suggest someone or something else needs to take Twitter’s place as a political tool.”

Professor Wu urged the company to use discretion: “Twitter needs to be careful not to be in a position where it’s no longer helpful to a rebellion against oppressive governments. It needs to remain its old self in some circumstances.”

Twitter’s policy of allowing its users to adopt pseudonyms made it particularly useful to many protest organizers in the Arab world, and its chief executive went so far as to call it “the free-speech wing of the free-speech party.”

But Professor Wu wondered aloud if the new policy would have allowed Egyptians to organize protests using the service.