The Athens Stock Exchange has fallen by about 23 percent as trading resumes after a five-week closure.
The country's top four lenders - Piraeus Bank, National Bank, Alpha Bank, and Eurobank - were all down by about 30 percent.
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.
Not long after the stock market reopened, it plunged to 615.16 points - down by 182.36 points from its closure on 26 June.
Banks account for about a fifth of the main Athens index.
Although Greece struck a bailout deal with its creditors last month, political in-fighting over the conditions could still result in Prime Minister Alexis Tsipras calling an early election.
The Greek economy has begun to reverse the gains it was making before Mr Tsipras' Syriza-led coalition took power in January on an anti-austerity platform.
The European Commission expects Greece to go back into recession this year, with the economy contracting by between 2 percent and 4 percent.
The Greek economy was in recession for six years until 2014.
- Reuters / BBC