Greece's President Karolos Papoulias says the country is on the "brink of the abyss", after three people died during protests over planned austerity measures.
As a general strike paralysed the nation, some demonstrators tried to storm the parliament and others hurled petrol bombs at shops and businesses in the capital, Athens.
A bank in the central city was set alight, killing three employees. The three victims - two women and a man - found their way blocked as they tried to escape to the roof, and suffocated.
Prime Minister George Papandreou told MPs in parliament the killings were a "murderous act".
Protesters are angered by spending cuts and tax rises planned in return for an international 110 billion euro bail-out for Greece, to prevent it defaulting on its debts, the BBC reports.
The protests happened during the third general strike in Greece in as many months.
Flights in and out of Greece stopped, trains and ferries were not running and schools, hospitals, and many offices were shut.
The strikes and protests have led to further falls in share prices on European stock markets and another sharp decline in the value of the euro.
Traders are worried that Greece's problems will drag down other vulnerable economies where the euro is the currency.
In the latest move, credit rating agency Moody's has warned it may downgrade its assessment of Portugal's ability to repay its debts.
In the United States, the Dow Jones industrial average dropped 0.54% to 10,868.12, on Wednesday, while the euro hit a 14-month low as investors shunned the debt of weaker euro zone countries.
All the major European markets fell, the NZX 50 shed almost 1.5% on Wednesday, and the Australian 200 Index closed 1.3% down on Wednesday.
