The main Athens stock index .ATG plunged in its worst ever one-day performance after only a few minutes of trading.
Banking shares, which make up about 20 percent of the index, were particularly hard hit. National Bank of Greece (NBGr.AT), the country's largest commercial bank, was down 30 percent, the daily volatility limit. The overall banking index .FTATBNK was also down to its 30 percent limit.
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The nearest blue chip futures contract expiring in August ATFQ5 traded down 20.5 percent, adding to losses of 15.2 percent at the open.
"Most of the selling pressure is seen in bank shares, where there is about 100 million euros worth of unexecuted selling orders," said investment adviser Theodore Mouratidis.
"There may be some more slide in store for (Tuesday) unless buyers emerge later in the session," he said.
Trading on the Athens bourse was suspended in late June as part of capital controls imposed to stem a debilitating outflow of euros that threatened to collapse Greece's banks and hurl the indebted country out of the euro zone.
Since then, Athens has agreed a framework bailout plan with its European Union partners in exchange for stringent reforms and budget austerity.
