Tens of thousands of people are expected to march in Madrid on Saturday against government austerity measures aimed at slashing the public deficit and avoiding the need for a financial bailout.
The demonstration has been organised by Spain's two leading trade unions, CCOO and UGT, along with roughly 150 smaller organisations, including groups representing students, immigrants and women.
"Over one thousand buses are expected at this demonstration which will be massive," UGT spokesman Fernando Navarro told AFP.
Among those expected to take part in the protest are teachers, nurses, firefighters, police officers and other public workers who have been especially hard-hit by the government spending cuts.
They will be joined by members of Spain's "Indignado", or "Indignant", movement, which has staged sit-in at public squares across the country against economic inequality.
The demonstrators will march in separate columns from different corners of the city under the slogan "They want to ruin the country" to the Plaza Colon square where they will converge at noon (1000 GMT).
Teachers, students and their supporters plan to march in green in their own column to the rally; nurses and doctors and other medical staff will wear white while public administration workers will wear orange in their column.
Pensioners and people suffering from disabilities also plan to take part.
In July, Prime Minister Mariano Rajoy's conservative government eliminated public workers' annual Christmas bonus, equivalent to a seven-percent reduction in annual pay, as part of austerity measures worth 102 billion euros ($126.5 billion) to be put in place by 2014 to reduce Spain's public deficit.
The government spending cuts have also led to reductions in staff levels at schools and hospitals and they follow a pay cut in public workers' salaries of an average of five percent in 2010.
Demonstration organisers argue the government austerity affects mainly the middle and lower classes and spare the wealthy and large companies. They want the austerity measures to be put to a referendum.
UGT head Candido Mendez said the Spanish people should be given the chance "to clearly say whether they are in agreement or not" with the spending cuts.
"It is not inevitable that that the markets govern us, that Spain gets a bailout for its economy," he added.
The government hopes the spending cuts it has put in place will prevent Spain from needing a multi-billion-euro bailout like the ones received by those Greece, Ireland and Portugal, which come with detailed conditions and regular inspections.
But with Spain facing a worsening recession, high borrowing costs and looming debt repayments including about 30 billion euros ($38 billion) in October, investors believe the country will soon seek a full-blown bailout.
Madrid has already accepted a eurozone rescue loan of up to 100 billion euros to save its banks, still reeling from a 2008 property market crash.
The last major march against government austerity measures took place on July 19 when hundreds of thousands of people marched through the streets of Madrid. Protests were held in over 80 Spanish cities that day.
The government is committed to lowering Spain's deficit to 6.3 percent of output this year from 8.9 percent in 2011, the third-largest in the eurozone.
It then wants to lower the deficit to 4.5 percent in 2013 and 2.8 percent in 2014, which will likely involve more painful spending cuts.