The Greek government under Prime Minister Antonis Samaras is attempting to persuade the International Monetary Fund, the European Commission and the European Central Bank, which makes up an international loan-managing trio called the troika, that the government's finances are on sound enough footing for the next multibillion-dollar aid installment.
Samaras, however, has recently asked for two-year extensions for Greece to meet terms of loans already made.
Greece needs another $40.7 billion to keep from going into default, The New York Times reported.
Samaras has agreed find $15 billion in a combination of spending cuts and new revenue, which would be the third round of austerity measures in as many years, the Times said.
The unions' strike is expected to include civil servants, teachers, professors, air traffic controllers, doctors, pharmacists and bank employees.
The only subway lines expected to operate are those that could bring protesters into Athens. Police have estimated 1 million will participate in the strike.
Slowdowns or shutdowns are expected in many businesses, including airports.