World leaders have urged for restraint as the unrest sparked by the death of an unarmed black man in police custody in the United States took global proportions.

Large-scale demonstrations and riots erupted across America after 46-year-old George Floyd died from asphyxiation when a white police officer pushed his knee into Floyd's neck for nearly nine minutes while detaining him in the city of Minneapolis on 25 May.

The chant "Black lives matter" has echoed across the world as mass rallies were held in major U.S. cities and in London, Berlin, Milan, Dublin, Toronto and in distant New Zealand.

The popular outburst led to a number of countries voicing their worry over the state of affairs in the U.S., especially in light of president Donald Trump threatening to send in the military to quell the violence and looting.

British prime minister Boris Johnson described the killing of Floyd as "appalling and inexcusable." Johnson supported the right to protest but added: "I also believe that protest should take place in a lawful and reasonable way."

Canadian prime minister Justin Trudeau expressed his horror at the turn of events and said "it is a time to pull people together, but it is a time to listen, it is a time to learn what injustices continue despite progress over years and decades."

Police officer Derek Chauvin was charged with third-degree murder and manslaughter. He has been newly charged with an additional, more serious count of second-degree murder, according to court documents filed in the case. 

Three fellow officers also dismissed from the Minneapolis police department along with Chauvin the following day were charged on Wednesday for the first time in the case – each with aiding and abetting second-degree murder and with aiding and abetting manslaughter. 

READ MORE: U.S.: All four police officers charged in George Floyd death

The European Union's top diplomat Josep Borrell backed the right to peaceful protest and called for a de-escalation of tensions.

A more scathing criticism was forthcoming from Turkish president Recep Tayyip Erdogan.

In a tweet he said: "The racist and fascist approach that led to the death of George Floyd in the U.S. city of Minneapolis as a result of torture has not only deeply saddened all of us, but it has also become one of the most painful manifestations of the unjust order we stand against across the world."

Iran's supreme leader Ali Khamenei said the incident shows the "true face" of the United States and its oppression of the peoples of the world, including its own.

"These are realities that have always been camouflaged or hidden, but they are not new. It's like the mud of a pond which rises to the surface," he added.

China's foreign ministry spokesperson Zhao Lijian said that the unrest shows how urgent it is for the U.S. to resolve relevant problems.