Battle-hardened servicemen and firefighters were reduced to tears Friday at the funeral of fallen Bravest and Marine Christopher Slutman, who was killed in combat in Afghanistan.

Midtown was a sea of blue as thousands of firefighters lined Fifth Avenue on the gray morning to salute Slutman, a man who friends described as a person of integrity who made the world a better place.

“Christopher was my best friend. Over 30 years, we came to know each other as brothers,” Marine Bruce Weaver told the congregation of mourners through tears.

“Christopher was everything right about this country. You knew he was there until the end. The world needs more Christopher Slutmans.”

The 43-year-old Marine staff sergeant — a 15-year FDNY veteran — was killed by a roadside bomb April 8 near Bagram Airfield, a US military base in Afghanistan. Two other members of Slutman’s Massachusetts-based Marine Reserve unit also were killed.

His casket arrived draped in an American flag at St. Thomas Church in Midtown atop a truck from Ladder 27 in the Bronx, where he was a firefighter.

He leaves behind wife Shannon and three young daughters.

Shannon clutched Weaver and sobbed as her husband’s casket arrived at the church where he was bid farewell by thousands of mourners.

Friends and colleagues described the Delaware-born man as a loving father, husband and friend who was committed to both the fire department and the armed forces.

Marine Sgt. Major Christopher Armstrong also spoke through tears as he described his heart “breaking” when he heard Slutman had been killed.

“There was a small number of men who when they spoke, silence fell. Christopher Slutman immediately stood out as a leader who was respected and admired,” he said.

“If I were given any mission anywhere on Earth, I would want Chris Slutman and his fallen brother Christopher Hynes by my side.”

Slutman’s casket was led up the Manhattan street by a procession of police cars, motorcycles and fire trucks while bagpipes from the FDNY’s Emerald Society Pipe & Drums played.

Slutman’s wife and daughters looked on as his casket was lowered from the truck by members of the FDNY before Marines picked it up and carried it into St. Thomas.

The casket was saluted by a massive gathering of firefighters and Marines before his family entered the church along with Mayor de Blasio, First Lady Chirlane McCray.

FDNY Battalion Chief Christopher Williamson fondly recounted a story about Slutman once ordering a pink milkshake while his firehouse brothers bought coffee.

“I said, ‘Jarhead Marines don’t drink pink milkshakes,’ ” said Williamson.

Slutman, sticking his nose deep into the plastic cup, emerged with a pink milk mustache and cream on his nose and said: “This one does.”

Firefighters maintained a guard of honor outside as heavy rain fell and soaked mourners.

Shannon Slutman left the church holding hands with her two youngest daughters.

When Slutman’s body was carried back into the Ladder 17 Engine, the middle daughter started to cry and tucked her face into her mother’s side.

A few moments later, an FDNY officer presented Shannon with her late husband’s fire helmet as she held back tears and grabbed onto her children.

As the procession drove away, the sound of a single bugle rang out along Fifth Avenue.

Slutman will be buried at Arlington National Cemetery in Virginia.