A man walks past a screen displaying a chart with the evolution of the Greek stock index in Athens, Greece, in this file photo. [Photo/IC]

ATHENS - Greece's stock market plunged nearly 23 percent on Monday when it opened after a five-week shutdown brought on by fears the country was about to be dumped from the euro zone.

The main Athens stock index 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, the country's largest commercial bank, was down 30 percent, the daily volatility limit. The overall banking index was also down to its 30 percent limit.

The nearest blue chip futures contract expiring in August 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.

But implementation of the deal is some way off, keeping alive political and economic stability concerns. There is also concern that Prime Minister Alexis Tsipras may need to call a snap election.

CONCERNS FOR FUTURE

The Athens General Index has fluctuated sharply this year, which has seen Tsipras's newly elected Syriza government wrangle with European Union and International Monetary Fund lenders in bitter bailout talks.

Traders had been expecting losses on Monday for a number of reasons. Negotiations on a new bailout might bog down, for example, leaving the government and banks perilously short of cash.

A report on Sunday in Avgi newspaper, which is close to Syriza, said the government was seeking 24 billion euros ($26.37 billion) in a first tranche of bailout aid from international lenders in August.

Of this, the newspaper said, 10 billion euros was earmarked for an initial recapitalisation of Greek banks, 7.16 billion euros to repay an emergency bridge loan and 3.2 billion euros to repay Greek bonds held by the European Central Bank and others.

The European Commission, however, believes an agreement in August is unlikely and that a new bridge loan will be needed.

This all puts Greek bank shares in the spotlight. Recapitalisation, both long- and short-term, waters down the value of existing shares. The banks are also not making profits this year and have lots of bad loans.

Similarly, Greeks themselves are being severely restricted. To limit the possibility of using shares as part of euro-flight, the government and ECB have said no extra money can be withdrawn by Greeks from deposit accounts to buy shares.

A Greek regulatory source also said the government would extend a ban on stock selling when it expires later on Monday.

Greece's dismal economic prospects may also weigh on the market. The European Commission says the Greek economy will shrink by 2 to 4 percent this year, a return to the recession that plagued the country for six years until 2014.

On Monday, a survey showed Greek manufacturing activity plunged to a record low as new orders plummeted and the three-week bank shutdown caused serious supply problems.