GREECE—Greece’s two largest union groups started a 48-hour walkout on Friday, in opposition to an austerity bill needed to unlock further rescue funding for the country, a vote on which is expected over the weekend.

The strike is set to bring public transport to a halt, close government offices and keep ships tied up at ports amid a media blackout in opposition to the tax and pension reforms bill.

The proposed law introduces the largest reorganization of the country’s pension system seen in recent years, by reducing benefits, raising social security contributions and introducing a new national pension.

It is a part of a package of measures Greece agreed to last summer in exchange for a third bailout with international creditors, eurozone governments and the International Monetary Fund, amounting to €5.4 billion ($6.2 billion), or 3% of gross domestic product.

Private-sector union group GSEE described the bill as being the final nail in the coffin for workers and pensioners.