The presidents from Azerbaijan, Kazakhstan, Georgia and Turkey were present for the ceremony at the Sangachal oil terminal on Wednesday, about 40km south of the Azeri capital, Baku, to open the taps for the first drops of oil to get through the Baku-Tbilisi-Ceyhan pipeline.
The pipeline from the Azeri capital to the Turkish Mediterranean port of Ceyhan is seen as a significant move towards reducing the West's dependence on Middle Eastern oil.
Most Caspian oil exports previously have moved through Russian pipelines.
The $3.2 billion project, with a capacity of one million barrels a day, is the first direct oil link between the landlocked Caspian, which is thought to contain the world's third-largest oil and gas reserves, and the Mediterranean.
Pipeline officials said it would take up to a month and a half to fill the Azerbaijani section of the pipeline.
The Georgian part will be ready after that, and then the Turkish stretch, which Turkish authorities have said should be filled by 15 August.
It will take about 10 million barrels of crude to fill the entire pipeline.