The Hamas leader Yahya Sinwar has for years overseen a secret police force in Gaza that conducted surveillance on everyday Palestinians and built files on young people, journalists and those who questioned the government, according to intelligence officials and a trove of internal documents reviewed by The New York Times.

The unit, known as the General Security Service, relied on a network of Gaza informants, some of whom reported their own neighbors to the police. People landed in security files for attending protests or publicly criticizing Hamas. In some cases, the records suggest that the authorities followed people to determine if they were carrying on romantic relationships outside marriage.

The Hamas leader Yahya Sinwar has for years overseen a secret police force in Gaza that conducted surveillance on everyday Palestinians and built files on young people, journalists and those who questioned the government, according to intelligence officials and a trove of internal documents reviewed by The New York Times.

The unit, known as the General Security Service, relied on a network of Gaza informants, some of whom reported their own neighbors to the police. People landed in security files for attending protests or publicly criticizing Hamas. In some cases, the records suggest that the authorities followed people to determine if they were carrying on romantic relationships outside marriage.

Everyday Gazans were stuck — behind the wall of Israel’s crippling blockade and under the thumb and constant watch of a security force. That dilemma continues today, with the added threat of Israeli ground troops and airstrikes.

“We’re facing bombardment by the occupation and thuggery by the local authorities,” Ehab Fasfous, a journalist in the Gaza Strip who appeared in the files of the General Security Service, said in a phone interview from Gaza.

The documents were provided to The Times by officials in Israel’s military intelligence directorate, who said they had been seized in raids in Gaza.

The General Security Service is formally part of the Hamas political party but functions like part of the government. One Palestinian individual familiar with the inner workings of Hamas, who spoke on the condition of anonymity because of the sensitivity of the matter, confirmed that the service was one of three powerful internal security bodies in Gaza. The others were Military Intelligence, which typically focuses on Israel, and the Internal Security Service, an arm of the Interior Ministry.

With monthly expenses of $120,000 before the war with Israel, the unit comprised 856 people, records show. Of those, more than 160 were paid to spread Hamas propaganda and launch online attacks against opponents at home and abroad. The status of the unit today is unknown because Israel has dealt a significant blow to Hamas’s military and governing abilities.

The Israeli intelligence authorities believe that Mr. Sinwar directly oversaw the General Security Service, according to three Israeli intelligence officials, who spoke on the condition of anonymity because they were not authorized to discuss the matter publicly. They said the slide show was prepared for Mr. Sinwar personally, though they did not say how they knew that.

The presentation said that the General Security Service works to protect Hamas’s people, property and information, and to support its leadership’s decision-making.

Some tactics, like amplifying Hamas’s own message, appeared to be routine politicking. In other instances, officials suggested using intelligence to undermine opponents and distort their reputations, though the files were vague about how that was to be done.

“Undertaking a number of offensive and defensive media campaigns to confuse and influence adversaries by using private and exclusive information,” the document read.

The General Security Service’s goals are similar to those of security services in countries like Syria that have used secret units to quell dissent. The files of the General Security Service, though, mention tactics like censorship, intimidation and surveillance rather than physical violence.

“This General Security Service is just like the Stasi of East Germany,” said Michael Milshtein, a former Israeli military intelligence officer specializing in Palestinian affairs. “You always have an eye on the street.”

“There are a lot of people practicing self-censorship,” said Mkhaimar Abusada, a professor of political science from Gaza City. “They just don’t want problems with the Hamas government.”

The files also show that Hamas was suspicious of foreign organizations and journalists.

When Monique van Hoogstraten, a Dutch reporter, visited a protest encampment along the border with Israel in April 2018, the authorities noted the most banal of details. They noted the make and model of her car and her license plate number. They said she took pictures of children and tried to interview an elderly woman Ms. van Hoogstraten confirmed the reporting trip in an interview with The Times.

None of the files reviewed by The Times were dated after the start of the war. But Mr. Fasfous said the government remained interested in him.

who told him to stop reporting and “destabilizing the internal front,” Mr. Fasfous recalled.

“I told him I was reporting on the truth and that the truth won’t hurt him, but that fell on deaf ears,” he said. “We can’t have a life here as long as these criminals remain in control.”