Aaron Kratzer is swaying to the rhythms of a New Orleans-style marching band here on Manhattan’s Avenue of the Americas, a famous avenue south of Central Park where on Sunday an estimated 300,000 people from around the globe gathered for the People’s Climate March, a parade and protest against the causes of climate change.

Mr. Kratzer, a community organizer who works with the First Congregational United Church of Christ in Corvallis, Oregon, also traveled across the country this week on the “people’s climate train,” he says, in which dozens of his West Coast peers traveled Amtrak’s California Zephyr from San Francisco to attend the massive march in Manhattan.

“We were even doing climate workshops all along the way,” says Kratzer as he marches amid the carnival-like parade of music, protest signs, and environmental chants.

The People’s Climate March, organized by an array of environmental, religious, and labor groups the past few months, was scheduled to coincide with the United Nations Climate Summit, in which nearly 100 heads of state, including President Obama, will discuss global carbon emissions and seek to create a framework for an international agreement at next year’s climate summit in Paris.

But environmental activists were stunned by both the size and energy of the raucous parade, which organizers say is the largest climate protest in history.

“After over forty years in the trenches of the environmental movement, I've never been more inspired and awe-struck,” said Frances Beinecke, president of the Natural Resources Defense Council in New York, in a statement from the parade. “The energy was electric and the turnout unprecedented. Today proves global support for climate action is undeniable. A swell of humanity has spoken as one: The time to act on climate is now.”

The parade began on the west side of Central Park and featured beating drums and dancers from indigenous peoples in Central and South American. They were followed by a cavalcade of union members, including hundreds under the blue United Auto Worker logo, many of whom carried the sign: “Climate change is real: Teach Science.” Thousands more community groups from around the world joined the march, each with identifying banners and political signs and slogans.

Ivan Ivanov, a student from Belgrade, carried a “Serbia loves the Environment” sign above his head as protesters around him chanted, “What do we want? Climate justice! When do we want it? Now!”

“I just feel we have to fight for the future,” says Mr. Ivanov, who spent the summer working in Cape Cod, Mass. He continued to march down to 42nd Street, where the parade passed through the city’s iconic Times Square.

Angel Hill, a student at Eastern Kentucky University in Richmond, Kentucky, joined about 50 others to take a bus to New York this week to join the People’s Climate March.

“Mountaintop removal is a huge problem in eastern Kentucky,” Ms. Hill says with a group of hundreds under “Protect Appalachian Communities” signs. “Where most of the coal fields and stuff are, you know, our water quality is terrible; you can't even touch your water sometimes because the wells are so bad.”

“And a lot of us wanted to come up here to New York and be a part of this,” Hill continues. “We thought it would be a huge part of history.”

Thousands of New Yorkers also joined the national and global contingents represented at Sunday’s march.

“I was a victim of Hurricane Sandy,” says Nelson Izquierdo, a counselor at the Borough of Manhattan Community College, and resident of Inwood, Long Island, where the superstorm wreaked havoc in 2012. “My house was destroyed, my car was destroyed, so I am definitely a big proponent of this climate march…. This is so much more than I thought it would be.”

At 1 p.m., as planned, the music and chants stopped for a moment of silence, which organizers said was for those who are most affected by climate change. Sarah Rivera, a high school student from the Bronx, held hands with fellow marchers and raised her arms during the startling silence that brought marchers to a quiet standstill on this stretch of Avenue of the Americas, normally abuzz with honking taxis, sirens, and loud-talking New Yorkers.

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The silence was followed by the “great big noise” – a whoop of cheers and shouts, as well as drums and horns and tolling bells from Manhattan churches.

“It’s really great to see all your peers, and people you go to school with, all out for one specific purpose,” says Ms. Rivera after cheering at the top of her lungs. “And now it’s just so empowering to see all these people come out here – from all over the place! It’s just a such a life-changing moment, for all of us, I think.”