Mass protests over the police killing of another black man in the United States spread globally in the past few days, as many demonstrators not only expressed solidarity with their American counterparts but denounced racism in their own countries.

Some critics, notably in China and Iran, used the killing to deflect from their own problems, saying it showed what they called the hypocrisy and arrogance of an increasingly isolated Trump administration.

The criticism thundered from the streets of Berlin, London, Paris and Vancouver, British Columbia, to capitals in Africa, Latin America and the Middle East. Artists drew an anti-racism mural in a besieged part of Syria. Lebanese and Chilean protesters offered advice on protection from police abuse.

The catalyst for the worldwide outpouring, George Floyd, 46, died last week after he was handcuffed and pinned to the ground by a white Minneapolis police officer, who has been charged with murder.

In London, thousands of demonstrators ringed 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, 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 what the city’s police chief called a “rather frantic” call about an assault.

And in Paris, among those calling for protests was the family of Adama Traoré, a 24-year-old black man who died in custody in 2016 after having been tackled by the police. 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.

“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 partly reflected what critics called the erosion of America’s moral authority. President Trump already faces criticism for a response to the coronavirus pandemic that has led the United States to relinquish its longtime role as a leader in times of crisis.

With Mr. Floyd’s death inciting protests in at least 140 American cities, images of street fights between police officers and protesters have spread swiftly across the world, drawing furious comments and calls for action.

Just as American demonstrators have protested 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 protested peacefully outside the U.S. Embassy on Saturday, some carrying signs that read, “Stop Killing Us.”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.

In downtown Montreal, a protest on Sunday turned violent after the police deemed it illegal. Clutches of protesters responded by throwing projectiles at the police, who used tear gas and pepper spray.

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

In China, where officials have been infuriated by Mr. Trump’s criticism of how they handled the coronavirus outbreak, the state-run news media featured reports about Mr. Floyd’s death and portrayed the protests as another sign of America’s decline. “BunkerBoy” became a trending topic after reports that Secret Service agents rushed Mr. Trump to a bunker on Friday night as hundreds of protesters converged outside the White House.

“Beijing could not have hoped for a better gift,” said Pierre Haski, a noted French journalist commenting Monday on France Inter. “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 moves 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, posted a doctored screenshot of a 2018 statement by American officials condemning Iran for corruption and injustice. In his version, references to Iran were replaced with America.

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

The head of the Addis Ababa-based African Union Commission, Moussa Faki Mahamat, said in a statement on Friday that Mr. Floyd’s death was a murder, and 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” mix of Covid-19, record unemployment and the “resurgence of America’s racial demons.”

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.” Such language is usually used for violent breakdowns in nations with few democratic or human rights safeguards.

The unrest also prompted 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 track 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.

Hundreds of Chilean protesters were injured or blinded by the bullets 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.

Prime Minister Scott Morrison said on Monday that while the video of Mr. Floyd’s death was shocking, Australians should not emulate 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; Catherine Porter from Toronto; Dan Bilefsky from Montreal; 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.