The United Nations designated Jan. 27, the day Soviet troops liberated the Auschwitz-Birkenau death camp in 1945, as a Holocaust remembrance day in 2005, Radio Free Europe reported.
BBC reported 1 million people died at the camp before it was liberated.
The United Nations said this year's theme was "Rescue During the Holocaust: The Courage to Care," honoring those who risked their lives to save Jews and other victims of the Holocaust.
BBC said 1,500 events are being held around the United Kingdom, including a ceremony at the Millennium Bridge in London.
Tanjug News Agency said a ceremony was held in Serbia at the Staro Sajmiste concentration camp memorial.
At the ceremony, Serbian parliament speaker Nebojsa Stefanovic said the crimes of the Holocaust must never be forgotten.
"I want to believe that a memory of the death and suffering of the victims will obligate new generations to stand up to every form of prejudice, chauvinism, anti-Semitism and irrational hate," Stefanovic said.