Civil servants, off-duty police and retired soldiers marched in Lisbon on Saturday against cuts imposed by the government under the terms of a 78 billion euro bailout by the EU and IMF.
They packed the Avenida Liberdade thoroughfare and the Restauradores Square after Parliament approved the first reading of a budget bill on Friday.
The demonstration was against cuts in miltary pay and benefits and reductions, in the number of military personnel.
Portugal earlier this year became the third euro zone country after Greece and Ireland to get a bailout.
Representatives of the European Commission, the European Central Bank and the International Monetary Fund are in Lisbon evaluating progress in implementing the bailout plan.
Portugal has to cut its budget deficit to 5.9% of GDP this year from 9.8% last year.
Next year, the gap has to be lowered to 4.5% of GDP and to 3% in 2013.
Prime Minister Pedro Passos Coelho said earlier that Portugal would have to put its finances in order itself and could not rely on the European Central Bank stepping in to pay the debts of countries that have failed to keep budget discipline.
A general strike against the austerity merasures is planned for 24 N ovember.
The final budget vote is due on 29 November. The coalition government has a solid majority in parliament.