"Indignant" activists, angered by a biting economic crisis they blame on politicians and bankers, vow to take to the streets worldwide Saturday in a protest spanning 71 nations.

It is the first global show of power by the movement, born 15 May when a rally in Madrid's central square of Puerta del Sol sparked a protest that spread nationwide, then to other countries.

As governments cut deep into welfare spending to try to trim huge sovereign debts, the protests have grown, and this weekend's demonstrations are being organised in Madrid, New York and around the world.

"United in one voice, we will let politicians, and the financial elites they serve, know it is up to us, the people, to decide our future," organisers said in a statement on http://15october.net/.

"We are not goods in the hands of politicians and bankers who do not represent us."

The organisers, relying heavily on Facebook and Twitter, say street protests will be held on 15 October in 719 cities across 71 countries in Europe, North America, Latin America, Asia and Africa.

The protests first took hold in Spain, with a jobless rate of 20.89 per cent, rising to 46.1 per cent for 16-24 year olds, where activists built ramshackle camps in city squares including Puerta del Sol.

Then they spread to Europe, finding strong backing in crisis-hit countries like Greece, and then worldwide – last month reaching the centre of global capitalism in Wall Street.

In Madrid, Saturday's protest will end in Puerta del Sol, still the spiritual centre of the overwhelmingly peaceful protests, even though the protest camp was dismantled in June.

Five marches will converge on the city's emblematic square of Cibeles at 6pm (1600 GMT) before proceeding to Puerta del Sol for assemblies lasting through the night, activists said Wednesday.

"They are deceiving the people, speaking about the public debt crisis to tighten our belts, cutting our rights and services," said a Spanish "indignant" activist, who only gave his first name, Manolo.

"The problem in Spain is the private debt," he told a news conference ahead of the global protest.

The Occupy Wall Street protest, which started 17 September with a camp of several hundred people in a small square in the New York financial district, has also struck a powerful chord among US media and politicians.

Organisers called a rally in Times Square for 5pm (2100 GMT), saying they would be at the centre of the international protests.

Iran's supreme leader, Ayatollah Ali Khamenei, predicted on Wednesday that the Wall Street demonstrations, which bring thousands of people together for marches, would one day spell the downfall of the West.

"This movement will soar to completely mark the downfall of the West and the capitalist regime," he said.

Anger over unemployment and opposition to the financial elite are common themes in the otherwise disparate movement.

But while Spain's protesters have specific demands, such as attacking unemployment by cutting working hours and imposing compulsory retirement at age 65, others are focussed on protesting existing conditions.

The outlook for the "indignants" is not clear.

French economist Thomas Coutrot, co-head of the ATTAC movement that seeks alternatives to market-ruled policies, said the indignant movement had a healthy "allergy" to being represented by any person or group.

"But it is true that it is not easy to build a movement without a representative," he added.