Greece's stock market suffered heavy losses on Monday, plunging nearly 23 percent at the open before recovering slightly, after a five-week shutdown brought on by fears the country was about to be dumped from the euro zone. One fund manager described it as "herd behavior" and said few people were buying. The main Athens stock index .ATG was down more than 18 percent in midday trading after an initial plunge that was larger than any one-day loss experienced on the bourse. By contrast, the broad European FTSEurofirst 300 index gained slightly .FTEU3.

Banking shares, which make up about 20 percent of the Greece 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.

Greece's banks have seen deposits severely depleted as Greeks have withdrawn their euros for fear the would be forcibly converted into a new drachma outside the euro zone. The banks have been propped up by emergency money from the European Central Bank. "Bank shares look like they have more room to slide on Tuesday before bids emerge," said the fund manager, who declined to be named. "It will take a few days for the market to balance out." Some companies outperformed, mainly those with exposure abroad, although they still fell. Greece's biggest telecoms operator OTE (OTEr.AT), along with jeweler Folli Follie (HDFr.AT) and aluminum producer Mytilineos (MYTr.AT), which are mainly exporters, saw their initial losses ease.