In many parts of the world, the death of yet another black man at the hands of the police in the United States is setting off mass protests against police brutality and reviving concerns that America is abandoning its traditional role as a defender of human rights.

On the streets of Berlin and Vancouver, in halls of power in Addis Ababa, Ethiopia, and Beijing, a chorus of criticism has erupted alongside the unrest in the United States over the death of George Floyd. Mr. Floyd, 46, died last week after he was handcuffed and pinned to the ground by a white police officer in Minneapolis. The officer who pressed his knee to Mr. Floyd’s neck has been charged with murder.

Paired with the global anger at American police violence in some cities has been another demand: that lawmakers heed the signs of racism and police abuse in their own countries.

In London, thousands of demonstrators gathered around the moated United States Embassy in defiance of stay-at-home coronavirus restrictions and chanted Mr. Floyd’s name, “I can’t breathe” and “No justice, no peace,” before making their way to Grenfell Tower. The building was the site of a devastating fire in 2017 that killed many Arab, Muslim and African residents. On a memorial at the base of the tower, a protester wrote, “Black Lives Matter.”

In Toronto, calls to end American racism merged with outrage at the recent death of Regis Korchinski-Paquet, 29, a black woman who the police said fell from her balcony after officers arrived at her home in response to reports of a “domestic incident.”

And in Paris, among those calling for a demonstration was the family of Adama Traoré, a 24-year-old black man who died in custody in 2016 after being tackled and pinned down by the police in the Paris suburbs. La Vérité Pour Adama, or “the truth for Adama,” an advocacy group led by Mr. Traoré’s sister, Assa, said Mr. Floyd’s death was a chilling reminder of Mr. Traoré’s.

“How can one not think of Adama’s terrible suffering when he had three police officers on him and he was repeating, ‘I can’t breathe’,” the group wrote on Facebook last week. “His name was George Floyd, who just like Adama died because they were black.”

The widespread condemnation reflected growing unease about America’s rapidly eroding moral authority on the world stage. President Trump already faces criticism across the globe for a response to the coronavirus pandemic that has led the United States to relinquish its longtime role as a global leader in times of crisis.

Now the death of Mr. Floyd has brought protests to at least 140 American cities, turning many into tear gas-filled battlefields. Images of police officers and protesters engaged in heated street fights have spread swiftly across social media sites around the world, drawing furious comments and calls for action.

Just as American demonstrators have been pained in part by the disproportionate toll of the coronavirus on black and immigrant neighborhoods, so, too, have activists around the world taken note of the gaping inequities laid bare by the pandemic. In England and Wales, for example, black people are twice as likely to die from the virus as white people.

In Berlin, thousands of demonstrators held a peaceful protest outside of the U.S. Embassy on Saturday, some carrying signs that read, “Stop Killing Us,” and graffiti artists sprayed Mr. Floyd’s image on a remaining stretch of the wall that divided the city for decades during the Cold War.

Three players in Germany’s top soccer league — the English forward Jadon Sancho, the French striker Marcus Thuram and the American midfielder Weston McKennie — made gestures of support for Mr. Floyd during weekend matches. Germany’s soccer federation, which bans players from making political statements during matches, said it would investigate the display by Mr. McKennie.

In Syria’s northwestern Idlib province, the target of a monthslong offensive by the Syrian government to seize control from opposition forces, two artists painted a mural on the shell of a ruined building that read “I Can’t Breathe” and “No to Racism.”

For America’s rivals, the tensions have provided an opportunity to deflect attention from their own problems.

In China, where officials have chafed at Mr. Trump’s criticism of they handled the coronavirus outbreak, the state-run news media heavily featured reports about Mr. Floyd’s death and portrayed the protests as another sign of America’s decline. The violent protests were covered extensively in the news media and on the social media platform Weibo. “BunkerBoy” became a trending topic after reports that Secret Service agents rushed Mr. Trump to a bunker on Friday night as hundreds of protesters gathered outside the White House.

Pierre Haski, a noted French journalist, commented on France Inter on Monday: “Beijing could not have hoped for a better gift. The country that designates China as the culprit of all evils is making headlines around the world with the urban riots.”

When an American official on Saturday attacked the ruling Communist Party on Twitter for moving to impose national security legislation to quash dissent in Hong Kong, a spokeswoman for the Chinese government fired back with a popular refrain among protesters in the United States.

“‘I can’t breathe,’ ” the spokeswoman, Hua Chunying, wrote on Twitter.

In Iran, Mohammad Javad Zarif, the foreign minister, accused the United States of hypocrisy. He posted a doctored screenshot of a 2018 statement by American officials condemning Iran for corruption and injustice. In his version, the references to Iran were replaced with America.

“Some don’t think #BlackLivesMatter,” Mr. Zarif wrote on Twitter.

The head of the African Union Commission, Moussa Faki Mahamat, said in a statement on Friday that Mr. Floyd’s death was a murder, and he criticized the “continuing discriminatory practices against Black citizens of the United States of America.”

Gilles Paris, Le Monde’s correspondent in Washington, wrote on Sunday that Mr. Trump was facing a “deadly spring” that has combined the Covid-19 crisis, record unemployment and the “resurgence of America’s racial demons.”

In an unusual statement on American affairs, the European Union said on Monday that it hoped “all the issues” related to the protests in the U.S. “will be settled swiftly and in full respect for the rule of law and human rights.” It usually reserves this type of language for violent breakdowns in nations with few democratic or human rights safeguards.

“We regret the loss of life, express our condolences to those affected and condemn violence and racism regardless of where it comes from,” the statement added.

Activists around the world vowed to continue to organize rallies and speak out about Mr. Floyd’s death. A protest was called for Amsterdam’s Central Dam Square later on Monday. The unrest in the United States also prompted activists abroad to offer advice to American demonstrators on how to keep the movement alive.

In Lebanon, a group compiled a document titled “From Beirut to Minneapolis: A Protest Guide in Solidarity” as a way to document state abuses. In Chile, the activist Danae Pradenas, writing on Twitter, cautioned demonstrators in the United States to protect their eyes from police rubber bullets, after Chile’s experience with hundreds of protesters who were injured or blinded by the police while protesting inequality last October.

An image of the Chilean flag with the message “I can’t see” and the United States flag with “I can’t breathe” is circulating on social media. The leftist legislator Gabriel Boric compared racism in the United States and in Chile against immigrants and Indigenous peoples on his Twitter feed, writing, “We are all George Floyd.”

In Australia, the hashtag #aboriginallivesmatter was trending on Twitter on Monday.

The images of unrest in the United States have reignited debate about Australia’s own troubles with police brutality. Some noted that more than 400 Indigenous Australians had died in police custody since 1991, without a single police officer convicted of abuse.

The relatives of David Dungay, an Aboriginal man who said “I can’t breathe” 12 times before he died while being restrained by prison guards in 2015, said they had been traumatized by footage of Mr. Floyd’s death, prompting them to call for another investigation into Mr. Dungay’s death.

With tensions rising, Prime Minister Scott Morrison said on Monday that while the video of Mr. Floyd’s death was upsetting and shocking, Australians should be careful not to adopt the destructive response seen in some American cities.

Mr. Morrison told a conservative radio station on Monday morning: “I saw a good meme on the weekend. Martin Luther King didn’t change anything by burning anything down or by looting any shops.”

To which many Australians quickly responded: You don’t understand Dr. King.

“What is with all these white people quoting MLK who’ve not read anything of King’s beyond a meme or seen anything beyond a 30-second YouTube clip of ‘I Have a Dream,’” Benjamin Law, an Asian-Australian writer and essayist, said on Twitter.

Reporting was contributed by Steven Erlanger and Matina Stevis-Gridneff from Brussels; Pascale Bonnefoy from Santiago, Chile; Melissa Eddy from Berlin; Damien Cave from Sydney, Australia; Thomas Erdbrink from Amsterdam; Vivian Yee from Beirut, Lebanon; and Aurelien Breeden and Elian Peltier from Paris. Albee Zhang contributed research.