Hundreds of youths pelted riot police with petrol bombs, bottles and chunks of marble in Athens today as yet another Greek anti-austerity demonstration descended into violence.

Police responded with volleys of tear gas and stun grenades in the Athens' Syntagma Square as protesters scattered during the clashes, which continued on and off for about an hour.

Four demonstrators were injured after being hit by police, volunteer paramedics said.

The Health Ministry said two of the protesters were treated in hospital and that their injuries were not serious.

A 65-year-old protester was also reported to have suffered a fatal heart attack during the demonstration.

The organisers of the protest march he participated in said the man had fallen ill before any rioting had broken out.

Authorities said around 70,000 protesters took to the street in two separate demonstrations.

It was the country's second general strike in a month as workers across the country walked off the job to protest against new austerity measures the government is negotiating with Greece's international creditors.

Hundreds of police had been deployed in the Greek capital ahead of the demonstration, as such protests often turn violent.

However, a protest march by about 17,000 people in the northern city of Thessaloniki ended peacefully.

The strike grounded flights, shut down public services, closed schools, hospitals and shops and hampered public transport in the capital.

Taxi drivers joined in for nine hours, while a three-hour work stoppage by air traffic controllers led to flight cancellations. Islands were left cut off as ferries stayed in ports.