"There are other things the [government] can do, before taking money from a retiree who earns 500 euros [$660] a month," Spyros Papaspyros, leader of the ADEDY civil servants' union, said.
The cuts will significantly affect civil servants' and pensioners' incomes and consumer taxes are to be increased.
Among the major measures announced on Sunday were a cut in bonus pay for civil servants and retirees; three years more for pension contributions; and the raising of the retirement age for women to 65, the same level as men.
Greece's socialist government is looking to push the measures, planned to be rolled out over three years, through parliament on Thursday. The government has a large majority in the house.
Schools, hospitals, tax offices and ancient sites will be closed on Wednesday.
Aircraft have been grounded, ferries have remained in docks and public transport has been halted due to the strikes.
About 4,000 teachers, garbage collectors, pensioners and civil servants rallied in Athens on Tuesday, in the first march organised by unions since the cuts were announced.
About 200 communists hung banners from the Acropolis, reading "Peoples of Europe, Rise Up" and a silent protest was held by the Greek navy.
"People are very angry, and many outside Athens don't realise what has happened,'' Spyros Antonopoulos, a 78-year-old retired civil servant, said.
"When they get their next check, they'll come to Athens with their children and grandchildren to protest.
"The way things are going, I won't buy any new clothes. I'll keep taking my trousers back to the tailor to get them patched up ... I'll wear them inside out if I have to."
Protests earlier this year have seen violence between demonstrators and police. During a May Day rally last Saturday, anarchists clashed with police who in turn fired tear gas.
Jyrki Katainen, the Finish finance minsiter, said that Greece's parliament needed to approve the austerity measures to prevent the crisis spreading.
"Our economies are so linked that a risk that problems spread from a country to another is very high.
"The markets have not calmed down ... because they fear that
national parliaments will not approve the package.
"The second risk is that Greece's parliament will not approve the economy repair plan and the third is how strong are the other countries, that is, is there a risk of spreading even if loans were granted?"
Fears that the crisis could spread caused a fall in Asian markets on Wednesday, as well as European and US markets on Tuesday.
The euro also fell to a one-year low against the dollar during Asian trading.