The main Athens stock index, the Athex, has fallen by 22.87% as trading resumes after a five-week closure.
The nation's top four lenders - Piraeus Bank, National Bank, Alpha Bank, and Eurobank - were biggest fallers, all down by 30%, the maximum allowed. Banks make up about a fifth of the index.
The bourse was shut just before Athens imposed capital controls at the height of the debt crisis.
Traders had predicted sharp losses as a result of pent-up trading.