By Joy Powell with Andrew Marszal in Los Angeles

MINNEAPOLIS, June 4 (AFP) — US protesters welcomed new charges brought Wednesday against Minneapolis officers in the killing of African American man George Floyd — but thousands still marched in cities across the country for a ninth straight night, chanting against racism and police brutality.

With a key demand met, demonstrators nevertheless staged large and mainly peaceful rallies calling for deeper change in cities from New York to Los Angeles, hours after the new indictments were announced.

Pressure on President Donald Trump mounted as his former Pentagon chief Jim Mattis launched a searing broadside, accusing the Republican leader of trying to "divide" America during the unrest.

In Minnesota, prosecutors had initially charged 44-year-old Derek Chauvin — the white officer filmed kneeling on Floyd's neck for nearly nine minutes — with third-degree murder.

But they said Wednesday they were upgrading the charge, roughly akin to manslaughter, to second-degree murder, which does not involve premeditation but carries stiffer penalties.

Chauvin's three colleagues at the scene of Floyd's May 25 arrest for allegedly seeking to buy cigarettes with a counterfeit bill are accused of being complicit in the killing.

Tou Thao, 34, J. Alexander Kueng, 26, and Thomas Lane, 37, were charged with aiding and abetting second-degree murder, and taken into custody.

The arrest of all four officers has been a focus for tens of thousands of protesters who have marched the streets of dozens of US cities, often defying curfews to condemn police brutality and demand racial justice. 

Former president Barack Obama applauded the "change in mindset" he sees among Americans demanding racial justice.

He urged the nation to "take the momentum that has been created as a society, as a country, and say 'Let's use this' to finally have an impact."

'Bittersweet moment'

US cities including Los Angeles and Washington delayed the start of their curfews by several hours Wednesday after looting and violence had subsided the previous night, while Seattle scrapped its curfew with immediate effect.

"For those peacefully demonstrating tonight, please know you can continue to demonstrate," tweeted Seattle mayor Jenny Durkan. "We want you to continue making your voice heard."

But several arrests were made in New York after groups of protesters continued to march in Manhattan and Brooklyn after the city's 8.00 pm curfew had passed.

In Manhattan, protester Brian Clark earlier said the charges were "a good start" but vowed demonstrators would "exercise our right to protest until every black person gets the justice they deserve."

"It's not enough," added fellow demonstrator Elijah B., who did not give his last name.

"This could have happened a week ago… it wasn't until people started marching on the streets and started tearing things that people started to pay attention."

A large group also protested at the US Capitol in Washington DC beyond curfew.

Thousands took to the streets in both Hollywood and downtown Los Angeles, where Mayor Eric Garcetti vowed to redirect $250 million toward black community health and education from budgets including the police department.

Floyd's family, in a statement thanking protesters, called the arrests and new charges a "bittersweet moment" — and a "significant step forward on the road to justice."

They urged Americans to continue to "raise their voices for change in peaceful ways."

'Law and order'

While condemning Floyd's death, Trump has adopted a tough stance towards the protesters, saying they include many "bad people" and calling on governors to "dominate the streets."

"We need law and order," he repeated on Wednesday. 

Mattis ripped into Trump, calling him "the first president in my lifetime who does not try to unite the American people — does not even pretend to try." 

"Instead, he tries to divide us," the retired Marine general said in a blistering statement posted online by The Atlantic.

Trump snapped back on Twitter, calling Mattis "the world's most overrated General."

The US president has raised the possibility of invoking the Insurrection Act to deploy active duty troops to quell unrest.

But Mattis' successor Mark Esper said that option should only be used as "a matter of last resort and only in the most urgent and dire of situations."

"We are not in one of those situations now. I do not support invoking the Insurrection Act," said the current defense secretary.

White House spokeswoman Kayleigh McEnany said Wednesday that the act remains "a tool available" to the president, who is facing a tough reelection battle in November.

"The president wants to protect America's streets," McEnany said, describing the criticism from Mattis as "a self-promotional stunt to appease the DC elite."

Trump meanwhile denied media reports that he was rushed for his safety to the White House bunker while protests raged in the streets outside. 

"It was a false report," Trump told Fox News Radio, before saying that he did go into the secure area for an "inspection" and only for a "tiny, little, short period of time."

Reports of Trump taking shelter sparked a wave of online mockery, which is believed to have contributed to his decision on Monday to make a controversial visit to a partly damaged church near the White House.

Police violently dispersed mostly peaceful crowds of protesters to clear a path for Trump, and the photo opportunity was widely condemned.