Past winners of the Nobel Peace Prize are in the Japanese city of Hiroshima to call for nuclear disarmament.
Hiroshima, devastated by an atomic bomb in 1945, was chosen to highlight the anti-nuclear message.
They are also to call for the release of jailed Chinese activist Liu Xiaobo, who was awarded the prize last month for his campaign for human rights.
Laureates noted Mr Liu's absence as well as Burma's detained pro-democracy leader Aung San Suu Kyi.
Mr Liu was sentenced to 11 years in prison last December after co-writing the Charter 08 document, which called for peaceful democratic reform in China.
The Chinese authorities called his Nobel award an "obscenity" and said Mr Liu would not be allowed to collect the prize in person in Oslo next month.
Mr Liu is being represented at the meeting in Hiroshima by his friend, Wu'er Kaixi, one of the student leaders of the 1989 Tiananmen Square democracy movement.
In a BBC interview, Mr Wu'er said that since Tiananmen, the rest of the world had failed to hold successive Chinese governments to account for their record on human rights.
The three-day meeting is being held outside of Europe for the first time to draw attention to the devastating power of nuclear weapons.
At the opening ceremony, laureates were given necklaces made of paper cranes - symbols of peace in Japan - by local school children.
A survivor of the Hiroshima attack, Akihiro Takahashi, who was a boy when the US dropped the bomb, addressed the meeting.
"I hate atomic bombs, but I know we cannot erase hatred by hating others. Hatred has to be overcome," he said.
Amid "growing concerns of a new global nuclear race and the threats posed by international terrorism, it becomes mandatory to find, and swiftly take, concrete actions in order to achieve global nuclear disarmament," a statement from the organisers said.
The Tibetan spiritual leader, the Dalai Lama, is leading the meeting. Former IAEA chief Mohamed Elbaradei and former South African president Frederik Willem de Klerk are also in attendance.
Last year's winner US President Barack Obama declined to attend but praised the efforts of the summit.
Former Soviet president Mikhail Gorbachev, who received the award in 1990 for his part in ending the Cold War, pulled out for health reasons.