PORT-AU-PRINCE, Haiti—In the days after Haiti’s president was shot to death, Jimmy Cherizier, leader of a powerful alliance of gangs here that calls itself the G9, issued an exhortation on YouTube, calling on his followers to rise up against the country’s oligarchs and seek justice for the assassinated leader.

“We are ready for war,” said Mr. Cherizier, a former police officer dressed in an olive drab military-style uniform and a camouflage baseball cap. “We are only warming up.”

Mr. Cherizier’s call to arms highlights a defining feature of modern Haitian politics: The ties between the Caribbean nation’s politicians and often-violent gangs that, according to the United Nations, human-rights groups and residents, effectively control swaths of the country.

U.N. officials warn the July 7 killing of the president, Jovenel Moïse, threatens to intensify what they say is already the worst wave of gang violence in years, making shootings and kidnappings part of daily life. The U.N. children’s agency Unicef says at least 18,000 people have been displaced by fighting, most since the beginning of June.

“The situation was bad before the pandemic but got worse during the pandemic and is now getting even worse because of the political situation and upscale in violence,” said Bruno Maes, Unicef’s Haiti representative. “We are only at the tip of the iceberg. This situation is worsening by the moment.”