The Duke of Sussex wants protection when he visits home and has claimed it's partly because an aggressive press jeopardises his safety and that of his family.
"This case is about the right to security and safety," attorney Shaheed Fatima said. "There couldn't be a right more important to any of us."
Fatima said the group that evaluated Harry's security needs acted irrationally and treated him unfairly by failing to follow its own policies that required applying a risk analysis about the duke's safety.
The three-day hearing in London's High Court is the latest in a string of Harry's legal cases that have kept London judges busy as he takes on the UK government and the British tabloid media.
Harry was not at Tuesday's hearing, set to be held behind closed doors after lawyers made opening statements.
Harry failed to persuade a different judge earlier this year that he should be able to privately pay for London's police force to guard him when he comes to town. A judge denied that offer after a government lawyer argued that officers should not be used as "private bodyguards for the wealthy".
Harry, the youngest son of King Charles III, said he did not feel safe bringing his wife, former actor Meghan Markle, and their two young children back to Britain and was concerned about his own safety after being chased by paparazzi following a London charity event.
Harry's animosity toward the press dates back to the death of his mother Princess Diana, who perished in a car wreck as her driver tried to outrun aggressive photographers in Paris.
Harry, whose wife is biracial, cited what he said were racist attitudes and unbearable intrusions of the British media in his decision to leave the UK.
The 39-year-old prince is challenging the decision by the Executive Committee for the Protection of Royalty and Public Figures to provide his security on a "case by case" basis after moving in 2020 to Canada and then California, where he and his family now reside.
He said the committee unfairly nixed his security request without hearing from him personally and did not disclose the panel's makeup, which he later learned included royal family staff.
He said Edward Young, the assistant private secretary to the late Queen Elizabeth II, should not have been on the committee because of "significant tensions" between the two men.
The Home Office has argued that any tensions between Harry and the royal household staff were irrelevant and that the committee was entitled to its decision because he had relinquished his role as a working member of the family.
The case is one of five that Harry has pending in the High Court.
The four other lawsuits involve Britain's best-known tabloids, including a case that alleges the publisher of the Daily Mail libelled him when it ran a story suggesting he had tried to hide his efforts to continue receiving government-funded security. A ruling is expected in that case on Friday.
Three other lawsuits allege that journalists at the Mail, the Daily Mirror, and The Sun used unlawful means, such as deception, phone hacking, and hiring private investigators to dig up dirt about him.
