Riot police wear face masks, as a precautionary measure against the COVID-19 coronavirus, allow a women to cross a cordon after a protest by pro-democracy demonstrators at a shopping mall in Hong Kong on May 9, 2020. (Photo by Isaac Lawrence/AFP Photo)

National Guardsmen are put through riot training in Boston's Commonwealth Armory on Friday, October 18, 1974. Massachusetts Gov. Francis W. Sargent called up the guard to quell school violence, but the city has been relatively calm and the guard has remained in the armories. (Photo by JWG/AP Photo)

Egyptian riot police charge towards protesters near Egypt's Interior Ministry, as violent demonstrations continued into their second day on February 3, 2012 in central Cairo, Egypt. (Photo by Ed Giles/Getty Images)

A protester uses molotov cocktails against riot police during May Day protests on May 1, 2010 in Athens, Greece. (Photo by Milos Bicanski /Getty Images)

People lay flowers for and mourn the anti-government protesters who were killed in the past weeks’ clashes with riot police.

A Palestinian demonstrator jumps in the air during clashes with Israeli riot police on May 13, 2011 in the East Jerusalem neighbourhood of Issawiya, Israel. Palestinians began marking the “Nakba” or “catastrophe” which befell them following Israel's establishment in 1948. (Photo by Uriel Sinai/Getty Images)

Riot Police form a cordon as a Police car burns on the opening day of the European Central Bank (ECB) in Frankfurt am Main, western Germany, on March 18, 2015. Supporters of the so-called Blockupy alliance consisting of social movements, activists, workers, trade unions and parties are expected to stage large protests against austerity and the authority of the European Central Bank when the bank's new headquarters officially will be on March 18, 2015. (Photo by Odd Andersen/AFP Photo)

A student aims a stick of wood at Bolivarian National Police officers during clashes outside the Central University in Caracas, Venezuela, Thursday, June 9, 2016. Students had planned to march from Venezuela's top university to elections headquarters, but hundreds of police in riot gear blocked the way. Students covered their faces with Venezuelan flags and threw bottles, stones and sticks while police lobbed tear gas. (Photo by Fernando Llano/AP Photo)