The national Students' Union of Spain called demonstrations in more than 50 towns on Thursday, its leader Tohil Delgado told AFP, predicting that thousands of students, pupils and teachers would join in despite looming exams.

A few hundred high school pupils demonstrated outside the education ministry in Madrid at noon on Thursday and organisers said a bigger march was planned to depart from Atocha train station at 6:00 pm (1600 GMT).

"We have called this demonstration as an initial response to this attack on public education, which is without precedent in the past 35 years," he said, citing reforms that protestors say are ruining their studies.

As Spain fights to stabilise its public finances, schools and universities have for months been complaining of shrinking budgets for research and extra work for teachers.

Since the last student marches on 29 February, the government has announced a further three billion euros in cost-cutting reforms, expanding class sizes and raising university fees to an average 1,500 euros from 1,000 euros.

The government says the measures are crucial to restart Spain's economy, now in its second recession since 2008 with 24-percent unemployment, but students say they are ruining their education and job prospects.

"There are about 40 people in each class and the teachers are tired," said one demonstrator, Paz Elena Fernandez, 16, adding that the public scholarships that many pupils in her working-class suburb rely on are being cut.

"Lots of people are abandoning their studies to find work, or are working while studying. But there's not much work going," she added. "Lots are neither studying nor working."

The latest demonstrations are part of a wave of anger at the austerity measures launched since the conservative Popular Party of Prime Minister Mariano Rajoy took over the government in December.

The government, which won an absolute parliamentary majority in last November's general election, has shown no sign of backing down from its reforms.

In a broader protest, the "indignant" movement on 15 May marks one year since it sprang up and occupied central Madrid for weeks with a tent city on the Puerta del Sol square.

The movement has promised four days of protests from 12 to 15 May.

The "indignants" appear divided over their internal organization, however, as traditional labour unions take centre stage, mobilising huge protests.

Hundreds of thousands of people demonstrated during a general strike on March 29, and a strike across the whole education sector is planned for May 22.

"We will not pay for your crisis!" yelled the first demonstrators in Madrid on Wednesday. "If there is no solution, there'll be a revolution!"