Foot and vehicle traffic is now frequently snarled by barriers and closed streets in the area, where the prospect of seeing Mr. Trump or another national political figure is a lure for visitors and a headache for high-end shops on the surrounding blocks.

“It is a high-density neighborhood and street traffic easily obstructs pathways to and from the building, making it profoundly challenging for the NYPD to establish a secure perimeter,” Mr. de Blasio, a Democrat, wrote, noting that the challenges affected not only Mr. Trump, but other residents of Trump Tower as well as the crowds that pass nearby.

The mayor said the number of officers involved in the round-the-clock security operation — from traffic agents to uniformed officers — was “classified,” but that most were working on overtime, at an extra cost to the city.

The letters, which Mr. de Blasio held up at the news conference — whose primary focus was a monthly police statistics briefing — did not include figures for the cost of security because, a spokesman for the mayor explained, the request would also include continuing security for Mr. Trump after the inauguration and for his wife, Melania, who plans to remain in New York City at least until the couple’s 10-year-old son, Barron, finishes the school year.