Zardari, addressing a crowd at the Bhutto family mausoleum Monday, said he wants to repair a "historical wrong" of the "judicial murder" of Bhutto, who founded of the Pakistan People's Party, Geo News reported.
Bhutto was Pakistan's fourth president and served as its ninth prime minister from 1973 to 1977. He was hanged in 1979 at age 51 for the murder of a political foe following a trial many considered a sham.
Thousands of people gathered Monday in Garhi Khuda Bakhsh, where the main celebration of Bhutto's death anniversary was to be observed, with other ceremonies held across the country.
"Bhutto is the symbol of courage, he struggled for the democracy in Pakistan and sacrificed his life for the same cause," Senate chairman Farooq H. Naek said.
Interior Minister Rehman Malik said an investigation into Benazir's death has been prepared and its details would be released at a meeting of the Central Executive Council although he did not give a date.
"We want the history to record that a wrong was committed and it needs to be corrected," Zardari said in an Associated Press of Pakistan report.