LONDON — The United States government began laying out its extradition case against Julian Assange, the WikiLeaks founder, in court on Monday by arguing that he had put lives at risk and was no better than an “ordinary” criminal.

“Reporting or journalism is not an excuse for criminal activities or a license to break ordinary criminal laws,” James Lewis, a lawyer representing the U.S. government, told the court.

Mr. Assange has been indicted on 17 counts of violating the Espionage Act for his role in obtaining and publishing secret military and diplomatic documents and he could face as many as 175 years in prison if found guilty on all charges.

His lawyers will begin presenting their defense later in the week.

Mr. Assange’s appearance in Woolwich Crown Court was the latest twist in a saga that stretches back to 2010, when he began publishing secret American military and diplomatic documents that were provided by the former Army intelligence analyst Chelsea Manning, who was convicted at a court-martial in 2013 of leaking the documents.