Palestinian authorities said a grave site discovered at the Nasser hospital, the main medical facility in central Gaza, contained nearly 400 bodies
Hamdallah was visiting Gaza in order to attend the inauguration of a wastewater treatment plant and meet officials, reports Efe news agency.
Palestinian news agency WAFA said Hamdalla and Majid Faraj, the head of the Palestinian General Intelligence Service, survived the explosion which left seven other people injured.
Although no one has claimed responsibility for the explosion yet, the Palestinian National Authority blamed Hamas, the Islamist group that rules Gaza.