UNITED NATIONS, July 17 (Xinhua) -- The United Nations will mark Nelson Mandela International Day on Wednesday with a vow to continue struggle for equality.

"Nelson Mandela was a towering global advocate for justice and equality," said UN Secretary-General Antonio Guterres Tuesday in his video message for Nelson Mandela International Day, annually observed on July 18, which was inaugurated by UN General Assembly in November 2009, in recognition of his global contribution to the culture of peace and freedom.

Mandela, who was born on July 18, 1918 and died on Dec. 5, 2013, was the first democratically-elected president of South Africa and the country's first black head of state.

"Rarely has one person in history done so much to stir people's dreams and move them to action," the UN chief said. "That struggle for equality, dignity and justice continues."

In December 2015, the General Assembly decided to extend the scope of Nelson Mandela International Day to also promote humane conditions of imprisonment and to encourage societies everywhere to treat prisoners as a continuous part of society by adopting the revised UN Standard Minimum Rules for the Treatment of Prisoners, known as the "Nelson Mandela Rules."