More than 60,000 protesters marched in Greek cities on Wednesday, police said, as a two-day general strike began against a new austerity bill demanded by Greece's international creditors to avert bankruptcy.

The highest turnout was in Athens where over 52,000 people converged on central Syntagma Square, where parliament is located, in separate protests organised by unions but also joined by unaffiliated Greeks fed up with austerity cuts.

"I work in the private sector and I'm in danger of losing my job," said a 45-year-old woman who declined to be named.

"Our bosses are taking the opportunity of the crisis to cut wherever they can. I'm desperate. The government's measures are going from bad to worse without benefit for the country. We are all terrorised," she told AFP.

Another 15,000 people demonstrated in the second city of Thessaloniki, local police said, and another protest was held in Heraklion on the island of Crete, where vandalism on bank branches was reported.

Authorities in Athens threw a cordon of riot police buses and a steel fence in front of parliament and shutting down two metro train stations in the area.

"Forward people, it's now or never to throw out the government, the IMF and the EU," said a banner carried by leftist demonstrators.

"The government must fall now," said another borne by Communists.

Some 3,000 officers were stationed around the capital, with additional forces guarding possible targets of violence such as embassies and government buildings.

A police motorcycle patrol was pelted with stones in the working-class district of Kaisiariani as the central Athens protests kicked off, and one of the riders was hurt, a police source said.

Most of the country's professional classes joined the 48-hour walkout including civil servants, tax collectors, doctors, teachers, sailors and taxi owners while traders, petrol station operators and bakers also shut down their businesses in protest against the government's economic policies.

Many government buildings were blocked by public sector staff outraged by new pay cuts and layoffs on top of a prior state payroll trim last year.

"Take the memorandum and get out of here," read a sign strung across a health ministry building in central Athens, referring to the loan bailout deal with the EU and the IMF that saved Greece from default in 2010.

Air traffic controllers were also to stage a 12-hour work stoppage on Wednesday, forcing airlines to scrap or reschedule several flights.

The main unions, GSEE for the private sector and Adedy for civil servants, were heading demonstrations in Athens and Thessaloniki.

The new austerity bill includes collective wage amendments, a new civil service salary system and temporary layoffs for thousands of public sector staff.

The new cuts are demanded by the EU and IMF in return for the latest loans from a 110-billion-euro ($151-billion) rescue programme agreed last year.

The Greek state has enough money to pay its bills through mid-November.