BRUSSELS -- Tens of thousands of people marched here Thursday against austerity measures as European Union (EU) leaders were to begin a crucial summit to endorse a comprehensive response to the year-long debt crisis.

Some 20,000 people were expected to show up for the demonstration organized by the European Trade Union Confederation (ETUC) during the morning rush hour, according to local police.

The demonstrators wearing red jackets occupied a main road connecting the part of the Belgian capital that houses EU institutions to the city center, severely disrupting traffic.

The Belgian police created an extended security cordon around the building where EU leaders will meet later Thursday. The two metro stations in the EU district were closed for security reasons.

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Local residents were also told not to park their cars along the route of the demonstration, which was called to show people's anger at the austerity measures and economic reform plans introduced by EU governments to tackle the sovereign debt crisis in many of their countries.

The protests were "to say 'no' to austerity plans in Europe, no to cuts in wages and pensions, no to insecurity and unemployment, no to deregulation of labor standards and social regression, and no to rising social inequalities," the ETUC said on its website.

At the two-day summit, EU leaders are expected to put the finishing touches to a package of measures to deal with the debt crisis.

It includes an expansion of the EU's bailout fund, establishment of a permanent rescue mechanism in the eurozone, lower interest rates on bailout loans provided to Greece and Ireland, a new round of stress tests in the banking sector and reforms to improve economic competitiveness and convergence of eurozone economies.

Trade unions were particularly concerned about the so-called Pact for the Euro, or competitiveness pact in its original name as tabled by Germany and France. The pact would see eurozone governments push ahead with painful reforms and co-ordinate their economies more closely to boost competitiveness.

Eurozone countries such as Belgium would be asked, when necessary, to review wage indexation mechanisms, which lead to automatic pay rises based on inflation.

Trade unions have been complaining the pact would undermine many cherished workers' rights and workers should not be forced to pay for the mistakes of the financial sector and governments.