World leaders on Wednesday paid tribute to and mourned the passing of US Senator Edward Kennedy, who died at 77 after a yearlong struggle with brain cancer.

US Senator Edward Kennedy (D-MA) waves as he walks out of Massachusetts General Hospital in Boston, Massachusetts, in this May 21, 2008 file photo. [Xinhua]

Edward Kennedy, nicknamed "Teddy", was the brother of former US President John Kennedy who was assassinated in 1963. He was the last of Kennedy brothers in the legendary family and known as "The Lion of the Senate."

Upon learning Kennedy's death, UN Secretary-General Ban Ki-moon extended "deep sympathies" to Kennedy's family and spoke highly of Kennedy's strong and firm support for "so many of the principles of the United Nations."

"He was not just a friend of those of power and high position, but even more to those who had neither," Ban said in a statement. "He was a voice for those who would otherwise go unheard, a defender of the rights and interests of the defenseless."

UN High Commissioner for Refugees Antonio Guterres said the life of the late senator and brother of former US President John F. Kennedy is a testimony to the difference a single policy-maker can make.

The UN refugee chief praised Kennedy for keeping "the plight of refugees on the international and national agenda," and "promoting policies and laws that saved and shaped countless lives."

For nearly five decades in the US Senate, Kennedy fought for legislation improving the treatment of refugees and asylum-seekers and reducing discrimination against them.

Three hours after Kennedy died, US President Barack Obama, who was on a vacation in an East Coast resort, said his family was "heartbroken" at the news.

US President Barack Obama embraces Senator Ted Kennedy (L) before Obama signed H.R. 1388, the Edward M. Kennedy Serve America Act, at the SEED Public Charter School in Washington, April 21, 2009. [Xinhua]

"An important chapter in our history has come to an end," Obama said in a statement. "Our country has lost a great leader, who picked up the torch of his fallen brothers and became the greatest United States Senator of our time."