"I can never forget this day for the rest of my life," he told tens of thousands of supporters in the Phnom Penh's Freedom Park,after flying into the country that morning.
"Without my colleagues, especially (party vice-president) Kem Sokha, we would not have this day," he said.
Estimates of the crowd that turned out to greet him varied from 50 to 100,000 people, the largest opposition rally in Cambodia's democratic history.
The atmosphere was festive as of 2 pm, with a strong police presence but no apparent hostility.