Thousands of people who were mourning at a George Floyd memorial in New York City are marching across Brooklyn Bridge.

They had gathered at Brooklyn's Cadman Plaza today for an event attended by Floyd's brother, Terrence, who addressed the crowd with calls for solidarity and peaceful protest in memory of his brother.

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"I'm proud of the protests," he said. "I'm not proud of the destruction."

Protesters take a knee before continuing their march on the Brooklyn Bridge after attending a memorial service for George Floyd. Photo / AP
Protesters take a knee before continuing their march on the Brooklyn Bridge after attending a memorial service for George Floyd. Photo / AP

It comes as a Florida police officer has been placed on leave with an inquiry pending after video footage showed him kneeling on a man's neck the week before George Floyd died.

More than 140 cities in the US and many more across Europe and around the world have seen people take to the streets. In many cities protests have turned violent with looting and property damage, as well as 13 people killed and more than 10,000 arrested.

'They bleed like you': Cop's emotional plea

NYPD Commissioner Dermot F Shea has given a heartfelt plea on behalf of police during the George Floyd protests.

"Understand please, they are human," he said. "They are dealing with the most difficult circumstances anyone can imagine. I don't think you can truly imagine it at times."

He said the NYPD was made up of 55,000 of the "greatest human beings you'll ever meet".

"They look exactly like you," he said. They look like you, they bleed like, you they cry like you, their tears fall off their faces like you."

He spoke of mistakes made by officers and apologised.

People gather outside Frank J. Lindquist Sanctuary at North Central University after a memorial service for George Floyd. Photo / AP
People gather outside Frank J. Lindquist Sanctuary at North Central University after a memorial service for George Floyd. Photo / AP

"Sometimes even the best, and the NYPD is the god-damn best police department in this country, fall down," he said.

"So for our part in the damage to civility, for our part in racial bias, excessive force, unacceptable behaviour, language and many other mistakes, we are sorry. Are you?"

Mysterious piles of bricks appearing in US cities

Conspiracy theories are flying around after huge piles of bricks have appeared in planned protest areas in the US this week.

Pictures and videos from Los Angeles, New York City, Kansas City, Dallas and Fayetteville in North Carolina all appear to show piles of bricks unattended in the middle of protests.

The footage has led to suggestions the slabs were either planted by police or by extremists to overshadow peaceful protests.

Police in Kansas City said in a tweet that they had discovered large piles of bricks and rocks that had been placed strategically to "be used in the riot".

"If you see anything like this, you can text 911 and let us know so we can remove them. This keeps everyone safe and allows your voice to continue to be heard," the Kansas City Police Department wrote.

Rapper Ice T, who has spoken out on police brutality many times, posted pictures of bricks in other cities.

"Looks like a set up to me," he wrote. "There's ALWAYS more than meets the eye."

It comes as US officials say they are investigating whether extremist groups have infiltrated police brutality protests.

Thousands pay tribute at George Floyd memorial

Hollywood celebrities, musicians and politicians gathered in front of the golden casket of George Floyd at a fiery memorial for the man whose death at the hands of police sparked global protests.

A huge crowd gathered outside the church to pay their respects as a civil rights leader declared it is time for black people to demand, "Get your knee off our necks!"

The service – the first in a series of memorials set for three cities over six days – unfolded at a sanctuary at North Central University.

The American ABC reported that there were more white people gathered outside the sanctuary than African Americans and that the crowd was "hanging on every word".

Just a few blocks away, a judge set bail at $750,000 each for the three fired Minneapolis police officers charged with aiding and abetting murder in Floyd's death.

Floyd, a 46-year-old out-of-work bouncer, died on May 25 after a white police officer, Derek Chauvin, put his knee on Floyd's neck for several minutes as he lay handcuffed on the pavement, gasping that he couldn't breathe.

Chauvin has been charged with murder, and he and the others could get up to 40 years in prison.

"George Floyd's story has been the story of black folks. Because ever since 401 years ago, the reason we could never be who we wanted and dreamed to be is you kept your knee on our neck," the Rev. Al Sharpton said in a fierce eulogy.

"It's time for us to stand up in George's name and say, 'Get your knee off our necks!"'

Rev Sharpton vowed a movement to "change the whole system of justice".

"Time is out for not holding you accountable! Time is out for you making excuses! Time is out for you trying to stall! Time is out for empty words and empty promses! Time is out for you filibustering and trying to stall the arm of justice!" he said.

Among the celebrities in attendance were TI, Ludacris, Tyrese Gibson, Kevin Hart, Tiffany Haddish and Marsai Martin.

George Floyd was held down by Minneapolis police officer Derek Chauvin pressing his knee into Floyd’s neck for 8 minutes and 46 seconds in total. Video / Darnella Frazier via AP

"All these people came to see my brother," Philones Floyd told the crowd at the memorial in awe as he recounted their childhoods playing catch and eating banana-mayonnaise sandwiches.

"That's amazing to me that he touched so many people's hearts because he touched our hearts."

The casket was flanked by white and purple flowers, and a vibrant image was projected above the pulpit of a mural of Floyd painted at the street corner where he was seized by police on suspicion of passing a counterfeit $20 bill at a convenience store.

The message on the mural: "I can breathe now".

- with AP