The city is constantly vigilant against possible terrorist attacks given its history but the mayor said the threat has changed significantly given the resources that a state such as Iran has compared to non-state organizations like al Qaeda or ISIS.
"It's just a world of difference," de Blasio told a news conference.
"We have to assume this action puts us in a de facto state of war," the mayor said.
De Blasio and senior police officials called the news conference as Iran promised vengeance in response to a U.S. air strike in Baghdad on Friday that killed Soleimani, the architect of Iranian influence in the Middle East.
The New York Police Department has intelligence and counterterrorism divisions and officers stationed at 14 foreign posts, all of which have been well-funded since the attacks of Sept. 11, 2001, that destroyed the Twin Towers of the World Trade Center.
New York officials consider the city the most enticing target for a terrorist attack in the U.S. and police go on heightened alert whenever there is political violence against civilians in the United States or abroad.
Police Commissioner Dermot Shea said officers were already on "heightened vigilance" and that New Yorkers could expect to see more uniformed officers, some heavily armed with long guns, at sensitive areas.
He did not specify what those were but a heavy police presence is typically seen on the subway, other mass transit, and tourist attractions when police go on heightened alert.