Ukrainian President Viktor Yushchenko was hosting the service at the ravine where the mass killings took place outside the capital, Kiev. Israeli President Moshe Katsav and other Jewish leaders are also attending Wednesday's events.
The service is to be followed by a forum on xenophobia and anti-Semitism, organized by Ukrainian authorities and Jewish Holocaust experts.
Mr. Yushchenko opened an exhibition on the massacre Tuesday, and described the Holocaust as a "deep wound" for all nations.
The massacre began on September 29, 1941, when Nazi forces who had just occupied Ukraine's capital ordered its Jewish residents to gather at a ravine.
Many Jews believed they would be evacuated. But, the Nazis instead began two days of executions, killing about 33,000 Jews and throwing their bodies into a large pit.
Over the next two years, the Nazis killed tens of thousands of other people at the site, including Jews, Roma (Gypsies) and prisoners of war. Historians believes as many as 100,000 people were killed.
Nazi forces exhumed and burned many of the bodies in 1943 before retreating from Ukraine as the Soviet army approached.S
ome information for this report was provided by AFP, AP and Reuters.