Thousands of people turned out Tuesday for marches in this capital and other Spanish cities to protest an "antidemocratic" push by the two main political parties for a constitutional amendment that would limit future budget deficits.

The proposal is the fruit of a pact between the governing Socialists and the conservative main opposition Popular Party, currently leading in the polls for the Nov. 20 general elections.

While many are demanding a referendum on the amendment, the Socialists and the PP say holding a plebiscite would make it impossible to conclude the process before the current Parliament dissolves at the end of this month.

The lower house of Parliament voted 316-5 last week to approve the amendment and the Senate plans to take up the text on Wednesday.

Tuesday's nationwide protests included Spain's two largest labor federations, leftist groups and elements of the May 15th Movement, which was behind massive mobilizations earlier this year to demand "real democracy."

The march in Madrid ended at the emblematic Puerta del Sol with the reading of a manifesto that said by trying to rush through a constitutional change in a mere two weeks, the Socialists and PP are "robbing the citizens of the possibility of having their say."

The document called the move a "serious attack" on popular sovereignty and a rupture with the consensus that formed the basis for Spain's 1978 constitution.

As for the substance of the amendment, the manifesto said it would endanger the welfare state and stressed that Spain's present large budget deficit is due as much to 15 years of declining taxes on the wealthy as to the 2008 financial crisis and the bursting of a real estate bubble.

Large protests also took place Tuesday in a score of other Spanish cities, including Barcelona, Valencia, Valladolid, Logroño and Palma de Mallorca.

The text of the constitutional amendment calls for a deficit approaching zero by 2020, but leaves the details for a subsequent piece of legislation to be debated and passed by the Parliament elected in November.

Senators agreed that Wednesday's session to consider the amendment will begin with a vote on a measure to submit the measure to a referendum.

The speed of the process and suspicions that the measure is being dictated to Madrid by the European Central Bank and by Germany - the only European Union country with a constitutional deficit limit - have spurred criticism even in the heart of the Socialist party.

Prime Minister Jose Luis Rodriguez Zapatero, who announced months ago that he would not seek a third four-year term, insists the amendment will send a positive message to financial markets.

Unlike some other heavily indebted EU members, Spain, with a jobless rate above 20 percent, is not a chronic deficit-spender and its current shortfall can be largely attributed to emergency responses to the financial meltdown.