The entire rail network will close, buses and trams will be idled, many schools will be shut and production at the Audi and Volvo car plants will stop.

Charleroi Airport, a hub for Ryanair and other low-cost carriers, was forced to scrap all flights due to union plans to block the access road. Prospects for flights at the country's main airport, outside Brussels, were unclear.

High-speed international train services, such as the Eurostar from London and Thalys from Paris, were not running into or out of the country from late on Sunday.

Unions have called Belgium's first general strike since 1993 over government plans to raise the effective retirement age along with other measures designed to save 11.3 billion euros ($14.84 billion).

The government also froze 1.3 billion euros of spending at the start of the year after a warning from the European Commission that it was not on track to meet its targets.

Belgium has pledged to bring its public sector deficit below the EU limit of 3 percent of gross domestic product this year to avoid an EU fine and to reassure investors that it has its finances under control.

Credit agency Fitch followed Standard & Poor's and Moody's on Friday in cutting Belgium's rating with a negative outlook, citing concerns of a worsening debt crisis and a recession that could undermine the commitment to reform.

The strike coincides with the 17th EU summit in two years as the bloc battles to resolve its sovereign debt problems. The EU leaders will sign off on a permanent rescue fund for the euro zone and are expected to agree on a balanced budget rule in national legislation.

BATTLE AHEAD

Belgium's government already knows growth this year will be below the 0.8 percent assumed for the budget drawn in December. A likely stagnation or contraction will force it to seek further savings when it revises the budget next month.

Economists estimate it will need to find an extra 1.5 billion to 2 billion euros.

The battle lines are already being drawn for that debate, with the French-speaking Socialist Party of Prime Minister Elio Di Rupo insisting that the rich should bear a greater burden with higher rates of tax on capital.

Pro-business Liberals and centre-right Christian Democrats, also in Di Rupo's six-party coalition, say higher taxes would push the country into a recession and that government spending can and should be cut more.

Union leaders say they fear the government might be tempted to suspend its system of wage indexation, the linking of pay to inflation criticized by the European Commission and international economic organizations as driving up prices and undermining Belgium's competitive position.

Economists say a single skipping of an automatic pay hike could save the government at least 1 billion euros.

For now, many Belgians appear to have accepted the need for austerity measures. According to an opinion poll in top-selling newspaper Het Laatste Nieuws last week, only 21 percent of Belgians supported the strike.

(Editing by Alessandra Rizzo)

By Philip Blenkinsop