Del Ponte began her opening statement by accusing Serbia of failing to arrest Bosnian Serb commander Ratko Mladic.
"Unfortunately, two men who should be sitting in this courtroom are still at large. I am talking of course about Ratko Mladic and Radovan Karadzic," del Ponte said in court as she outlined the prosecution's case.
"It is absolutely scandalous that they have not been caught. Serbia is fully capable to arrest them, but has refused." she said.
Of the seven indictees in the biggest at the tribunal, five are accused of genocide - Vinko Pandurevic, Ljubisa Beara, Vujadin Popovic, Drago Nikolic and Ljubomir Borovcanin.
The two other men on trial, Radivoje Miletic and Milan Gvero, are charged with crimes against humanity and violations of the laws or customs of wars including murder, persecution, forcible transfer and deportation.
All seven pleaded not guilty.
Del Ponte pledged to bring all those still on the run to trial and reiterated her criticism of Serbia for failing to arrest and transfer Mladic and Karadzic:
"Words cannot convey the magnitude of crimes committed and the suffering of the victims," Del Ponte said.
The massacre in the Bosnian Muslim enclave in eastern Bosnia is Europe's worst atrocity since World War Two. The UN war crimes tribunal has already convicted six people over the Srebrenica massacre. In 2001, Bosnian Serb officer Radislav Krstic was the first to be convicted of genocide for Srebrenica by the tribunal.