LONDON — Julian Assange, the founder of WikiLeaks, appeared in a British court on Thursday for an initial hearing on whether he will be extradited to the United States to face prosecution in connection with one of the most serious leaks of classified material in American history.Mr. Assange, 47, made a brief appearance by video link in Westminster Magistrates Court in London from Belmarsh Prison in another part of the city. A day earlier, he had been sentenced to 50 weeks in prison for bail-jumping.The hearing on Thursday lasted a just a few minutes, in which Mr. Assange told the judge that he did not wish to surrender himself to be prosecuted in the United States for what he called “journalism that has won many awards,” according to The Associated Press. His next hearing, in what promises to be a long extradition fight, is scheduled for May 30.The American indictment against him stems from a leak in 2010 of hundreds of thousands of classified documents, mostly related to the wars in Iraq and Afghanistan, that proved damaging and embarrassing for the United States and its allies. Mr. Assange faces a charge of conspiring with the former Army intelligence analyst Chelsea Manning to hack into a Pentagon computer network, a crime punishable by up to five years in an American prison.Around two dozen activists gathered outside the court to protest against his potential extradition, waving signs that read “Free Julian Assange” and “Is this all just about shutting us all up?” as they demanded his release.The case has fueled debate about whether the prosecution of Mr. Assange infringes on the American Constitution’s First Amendment right to freedom of speech. He says that he acted as a journalist in publishing material leaked by Ms. Manning to WikiLeaks and that he had nothing to do with the hacking.But the Justice Department says Mr. Assange helped Ms. Manning break a code to gain access to the classified network.Ms. Manning was convicted of espionage in an American court and received a 35-year prison sentence. She spent nearly seven years behind bars before her punishment was commuted by President Barack Obama in 2017.Legal experts say that Mr. Assange could face additional charges if extradited but that the extradition process could take years. The WikiLeaks founder has long fought against being transferred to the United States, citing comments from officials in Washington calling for the death penalty to be considered for his crimes.Mr. Assange holed up in the Ecuadorean Embassy in London for seven years to avoid being extradited to Sweden to face sexual assault allegations. He called himself a political refugee, claiming that Sweden would send him to the United States.Swedish prosecutors have not closed the door on charges there.Mr. Assange was arrested last month after the government of Ecuador suspended the asylum and citizenship that it had granted him.Speaking outside the courtroom on Wednesday, a day before the hearing on Mr. Assange’s potential extradition to the United States, Kristinn Hrafnsson, WikiLeaks’ editor in chief, called it the start of a “big fight.”“What is at stake there could be a question of life or death for Mr. Assange,” he told reporters. “It is also a question of life and death for a major journalistic principle.”Cristina Navarrete, 66, was among the crowd of supporters outside the court on Thursday. She called the hearing “a mockery” because no members of the public were allowed into the courtroom, though some journalists were admitted.“They chose a small court on purpose, it’s basically a secret trial,” Ms. Navarrete said in the packed foyer outside the guarded doors of the court.“If he is extradited, we’ll be very disappointed in the British justice system,” she added.Ben Brandon, a prosecutor working for the United States government who was in the Westminster court on Thursday, said that even if Mr. Assange were to complete his British prison sentence before a decision on the American extradition request, he would remain in custody. In that situation, he could apply to be released on bail until a final decision on extradition, Mr. Brandon said.After the hearing, Jennifer Robinson, Mr. Assange’s lawyer, said the American charge would cause a “massive chill on investigative journalism.”“No democratic nation would behave this way,” she said.