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 documents show that Hamas leaders, despite claiming to represent the people of Gaza, would not tolerate even a whiff of dissent. Security officials trailed journalists and people they suspected of immoral behavior. Agents got criticism removed from social media and discussed ways to defame political adversaries. Political protests were viewed as threats to be undermined.