A police rookie whose “dream was to be an NYPD officer’’ was killed when a department van full of newly minted cops heading for duty crashed in The Bronx on Sunday.

Police Officer Mike Williams, the 25-year-old son of an upstate cop, was sitting in the rear of the van, slated to work security for the UN general assembly and massive climate march, when the vehicle crashed around 5:10 a.m., authorities said.

“[Williams’] father has probably seen it all, but he never expected to see something like this involving his own son,” said a grim-faced Police Commissioner Bill Bratton, who was attending a conference in Aspen, Colo., when he was informed that one of his officers had been killed in the line of duty and immediately flew home.

There were a total of nine officers — all with Operation Impact and from the 47th Precinct — in the van at the time.

Williams was ejected when the cop driver lost control as he rounded a sharp corner on the rain-slicked Bruckner Expressway in Hunts Point, officials said.

Neither Williams nor most of the other officers in the van were wearing seatbelts, sources said.

“At least seven [of the cops] were flying around like rag dolls,’’ a source said.

John Miller, the NYPD’s deputy commissioner of intelligence, said, “[Williams] was in the last row on the passenger’s side. He came out the side window.”

Williams was taken to Lincoln Hospital, where he was pronounced dead.

The other cops suffered non-life-threatening injuries.

At Lincoln Hospital, more than 70 fellow cops saluted the young officer’s body — draped in an NYPD flag — as it was removed and loaded into a department truck.

Williams’ distraught mother and father soon followed behind and were ushered into an NYPD vehicle.

“His parents said it was his dream come true to be a police officer,” said NYPD Chief of Department Phillip Banks III, referring to the rookie, who joined the department in January. “It was all he ever wanted to do.”

Banks added that William’s father, Michael Williams Sr., is a veteran upstate cop with for more than three decades on the job.

“We’re all in a lot of pain today to lose a good young man who was just starting out,” Mayor de Blasio said later outside Williams’ precinct in The Bronx.

“Officer Williams’ dream was to be an NYPD officer,” de Blasio said, noting that he and Bratton spoke at the young cop’s academy graduation earlier this year.

“It was a very hopeful and positive moment,” he said. “He came from a police family. This is what he wanted to do with his life.”

Bratton added, “His father, a 32-year veteran, is still active with the Carmel Police Department. So this tragedy is even more poignant meaning for them.”

Bronx resident William Soto said he was listening to a police scanner and immediately rushed to the scene after the call came over the radio.

“It was raining pretty hard when I got out here,” Soto said, 49. “I think they skidded in the turn. That happens all the time here.”

“The EMS was already here working on the driver, who was bleeding from the head,” Soto continued.

“They put a neck brace on him and kept working on him as they put him in the ambulance.”

“Another officer was sitting on the ground all mangled. I heard he was the one that was ejected.”

Four of the injured officers were taken to Lincoln Hospital, while four more went to St. Barnabas and Jacobi hospitals, officials said.

Two of the injured officers, Wesley Taveras, 25 and Francine Devalle, 29, remained hospitalized Sunday night but were expected to make a full recovery. The rest were treated and released.

A cousin of Devalle’s, Kenia Reyes 23, said her cop kin is devastated by Williams’ death.

“They were pretty close,” said Reyes, adding that Devalle had just graduated from the academy herself in June.

Reyes, who was at her cousin’s side in the hospital, said de Blasio visited Devalle and “just said that he was grateful that she was OK and that they were here for her, and if [she] needed anything, that her family has expanded and that they were so sorry for what had happened.

It’s just hard to accept that he’s gone. He was so new to the force — he was on his way to a long and great career. To have his life cut short this way is just tragic. - Michael Williams' aunt Karen Fransen

“He gave her a hug and embraced and kissed her on the cheek,” Reyes added. “He was really, really nice. He shook all of our hands.”

She said her cousin was “fine, just a few cuts and scrapes.

“Mostly she’s tired. laying there resting. … She just remembers everyone screaming,” Reyes said.

PBA President Pat Lynch said of the injured officers, “This morning, they woke up to do a job that’s important — to protect people’s right to protest. Say a prayer for [Williams’] family so they can get through.”

Bronx union trustee Joe Anthony added, “Even though he was only a cop for a short time, [Williams] was a brother to everyone in the precinct. The whole [47] precinct is devastated.”

Bratton said investigators are still trying to determine what caused the crash on the foggy Bronx highway but did not rule out weather conditions or the van’s speed as potential factors.

The dead officer’s devastated aunt, Karen Fransen, said a contingent of about 12 NYPD cops made the drive up to his parents’ home in Poughquag to try and console them.

She described her godson as a talented basketball player who played and studied at Keene State College in New Hampshire.

“It’s just hard to accept that he’s gone,” Fransen said,adding that her nephew had hoped to follow in his father’s footsteps. “He was so new to the force — he was on his way to a long and great career.

“To have his life cut short this way is just tragic,” she said. “You can’t imagine what we are going through.”

Meanwhile, police at the 47th Precinct lowered the American flag outside the station house to half-staff in honor of Williams.

More than a dozen firefighters from Engine Company 38 and Ladder Company 51 turned up to offer support to the grieving cops and help hang up a memorial bunting from the side of the building.

Additional reporting by Daniel Prendergast, Reuven Fenton and Priscilla DeGregory