Drivers are staying away from Midtown Manhattan if they can. Workers returning to the office are taking the subway or allowing extra time for the bus. Deliveries are delayed or canceled.

United Nations gridlock — expected to continue through Sept. 26 — is back.

The opening day of the 77th U.N. General Assembly on Tuesday brought out crowds of world leaders, foreign dignitaries and ambassadors, along with their black-car motorcades, in the largest show of diplomatic force on New York streets in the three years since the pandemic.

Dozens of street closures, detours and security checkpoints throttled traffic in Midtown Manhattan, where the United Nations is headquartered. Some disgruntled New Yorkers — including those on foot, bikes and scooters — scrambled into a maze of unplanned turns and workarounds to get to offices, schools and doctor’s appointments.

“My boss told me beforehand to look out; it’s going to be crazy,” said Nik Summers, 47, who works as a building porter near the United Nations.

And it was. Mr. Summers, who rode the subway from his home in the Bronx, arrived at work before 6 a.m. to find rush-hour traffic lined up outside his building while police officers swarmed a nearby cafe in a rush to get coffee before taking up their security posts.

It was one of the biggest tests yet for New Yorkers who crave the return to the normalcy of prepandemic life but who may have forgotten its cost. Traffic and congestion, which largely disappeared early in the pandemic, have since returned. And in some neighborhoods, they are worse than before as soaring car ownership and a pandemic boom in cycling, curbside dining and deliveries have resulted in more people vying for street space than ever before.

At major crossings to New York from New Jersey — including the George Washington Bridge and the Lincoln and Holland Tunnels — traffic has just about rebounded to prepandemic levels with 10,532,949 vehicles in June compared to 10,583,879 vehicles for the same month in 2019, according to the latest data available from the Port Authority of New York and New Jersey.

Within hours of the General Assembly convening on Tuesday, traffic had visibly slowed to a crawl through a broad swath of Midtown, from 42nd Street to 57th Street and from First Avenue to Fifth Avenue.

At some intersections, police officers and traffic agents waved drivers along, moving traffic and keeping tempers from flaring. Several drivers were forced to turn off East 46th Street and onto Lexington Avenue because they could not show diplomatic IDs.

Jaime Ayala, 41, a delivery truck driver, said he hunted for parking for a particularly long time because of the street closings. He ended up several blocks away from his delivery route and had to pile packages high on a cart to roll down the sidewalk.

“It’s crazy,” Mr. Ayala said of the traffic. “It’s out of my control. There is nothing I can do about it.”

City transportation and police officials had previously urged drivers and delivery workers to steer clear of the area, or at the very least, avoid prime traffic hours from 6 a.m. to 7 p.m. A list of street closings were posted on the police department’s website.

“We are letting New Yorkers know the key to avoiding the congestion is to try walking, mass transit or riding a bicycle to get to their destination,” Ydanis Rodriguez, the city’s transportation commissioner, said last week.

City officials also issued gridlock alerts — an annual warning of the days that the city will likely experience its worst congestion — for the entire week. The congestion warnings, which had long been issued for the busy holiday season in November and December, were expanded in 2019 to include the annual General Assembly session.

Around the U.N. building, lines of embassy sedans and S.U.V.s were squeezed along curbs, or double-parked, as the drivers greeted each other. “It’s the same, nothing new,” Francis Leckey, a chauffeur, said after dropping off South Africa’s ambassador in the morning.

“We have South Africa, we have Ghana, we have Gabon,” Mr. Leckey added as he surveyed the other cars. “Over there is Pakistan. Those who are there are Canada. And over there is Namibia. And Botswana.”

Still, it was not all traffic headaches. The U.N. session has traditionally been accompanied by meetings, parties and shopping that provide an economic windfall for the city.

Workers at restaurants and stores, many of which are still struggling because office workers and tourists have not fully returned, said they were excited to see the U.N. crowd.

On Tuesday, men and women wearing lanyards with diplomatic badges strolled around Midtown, speaking multiple languages and filling outdoor dining tables. Protesters were also out. Circling the area were two flatbed trucks with flashing digital screens that condemned human rights abuses.

Andrew Rigie, the executive director of the New York City Hospitality Alliance, an industry group, said that many Manhattan restaurants have seen increased bookings for dinners and parties this week.

“While it’s an inconvenience with shutting down streets, congestion and security, it’s a critically important event for the city and the restaurant industry,” Mr. Rigie said.

At the counter of a flower and coffee shop on Second Avenue, the owner, Remi Kim, was busy taking incoming orders for bouquets — including some that were to be delivered to the United Nations.

“It’s only a week, but we’ll take it,” Ms. Kim said. “It just feels like we’re getting back to normal life with the crowds and the traffic.”