Spain's feminist movement is pushing for changes to the law after a court acquitted five men of gang raping an 18-year-old woman at Pamplona's bull-running festival.

Protesters have filled streets across the country in response to appeals from women's groups, leading Spain's conservative government to announce it will consider changing rape laws.

An online petition calling for the disqualification of the judges who acquitted the men had gathered more than 1.2 million signatures by Saturday.

The issue also hogged the headlines of newspapers all around the country and an order of Carmelite nuns added their voices to the wave of condemnation of the court judgment.

The current law is founded "on a concept of violence based on a male experience", said Mariam Martinez Bascunan, a political science professor at Madrid's Autonomous University.

Under Spain's criminal code, evidence of violence or intimidation must exist for the offence of rape to be proved.

"This legal nuance, which is not always easy to establish, leads to the painful question of just how much a person needs to fight to avoid being raped without risking getting killed, and still get recognized as a victim of a serious attack against sexual freedom while ensuring that the perpetrators do not enjoy impunity," top-selling daily El Pais wrote in an editorial.

Spain's rape laws were thrust into the spotlight after a Pamplona court on Thursday acquitted the five men of sexual assault, which includes rape, but found them guilty of the lesser offence of "sexual abuse". They were jailed for nine years.

The men, aged 27 to 29, had been accused of raping the woman at the entrance to an apartment building in Pamplona on July 7, 2016, at the start of the week-long San Fermin festival, which draws tens of thousands of visitors.

The five, all from the southern city of Seville, filmed the incident with their smartphones and then bragged about it on a WhatsApp messaging group where they referred to themselves as "La Manada", or "The Pack" in English.

"It is indisputable that the plaintiff suddenly found herself in a narrow and hidden place, surrounded by five older, thick-bodied males who left her overwhelmed and unresponsive," the judges said in their ruling.

"The videos show the plaintiff surrounded and stuck against the wall by two of the accused... she has an absent grimace, and keeps her eyes closed," they added.

- 'Macho culture' -

Large crowds of mainly women marched in cities across Spain, including Madrid and Barcelona, on Thursday night after the court sentenced the men. In the northern city of Santander protesters blocked roads, public television TVE reported.

On Friday thousands of people demonstrated outside the Pamplona court where the judgment was made for the second day in a row, shouting "it's not sexual abuse, it's rape". Some held banners reading "sister, I believe you".

A petition launched by a 38-year-old Spanish woman calling for the disqualification of the judges gained more than 1.2 million signatures in just two days.

And a community of 16 Carmelite nuns in the Hondarribia monastery in the Basque country condemned the court ruling on Facebook.

"We live cloistered away, wearing a habit that reaches down to our ankles, we don't go out in the evening, we don't go to parties, we don't drink alcohol and we've undertaken a vow of chastity," the nuns said.

"And because that's our free choice, we will defend with all the means at our disposal ... the right of all woman to FREELY do the opposite, without them being judged, raped, threatened, killed or humiliated," they wrote.

State prosecutors said they would appeal the ruling.

Adriana Lastra, a top official with Spain's main opposition Socialist party, said the court ruling was "disgraceful".

"It's the product of a patriarchal and macho culture," she added.

The case was cited in signs carried by many women during massive demonstrations held in cities across Spain to mark International Women's Day in March, which were among the largest in Europe.

"Spain was a pioneer in terms of equality," Soledad Murillo, Spain's former Socialist secretary of state for equality between 2004 and 2008, told AFP when asked about the huge turnout at the demonstrations.

- Authorization to work -

Spain in 2004 passed a landmark law against gender violence and women's groups mobilised strongly against a proposed law which would have restricted access to abortion, prompting the resignation in 2014 of the justice minister at the time, Alberto Ruiz Gallardon, who had tabled the reform.

"Younger women became aware of what their grandmothers lost," said Murillo.

She cited as an example the fact that during General Francisco Franco's 1939-75 dictatorship, women needed the authorization of their husbands to work, as well as special authorizations "to leave the country or drive".

Spain's women's movement has as a result fully embraced the causes of so-called "fourth-wave feminism" such as the fight against street and workplace harassment and rape that has been popularized worldwide as part of the #MeToo movement, said Murillo.

The movement has also been boosted by Spain's tradition of protests, which has manifested itself in recent years most visibly with the Indignados movement against economic inequality and corruption which occupied public squares in 2011, said Bascunan.