Thousands of people, mainly women, gathered at noon outside Andalusia's regional parliament in Seville, where Vox will on Wednesday endorse a conservative coalition government made up the Popular Party and Ciudadanos, putting an end to 36 years of Socialist rule in Spain's most populous region.
The demonstrators chanted “Our rights are not negotiable”. Many wore purple or waved purple — a colour long associated with gender equality.
Further protests against Vox are scheduled to take place at 7:00 pm (1800 GMT) across Spain, including in the capital Madrid and Barcelona, the country's second-largest city.
The slogan for the protests called by feminist groups is: “No step backwards in terms of equality”.
Feminists are angered because Vox has made scrapping tough laws against gender violence one of its main priorities.
The party says these laws encourage unfounded complaints against men by women “without scruples”.
Vox, which also campaigns against illegal immigration, has also called for an end to “subsidised radical feminist associations”.
The party won a surprise 12 seats in Andalusia's parliament in a regional election last month.
It initially made scrapping Spain's gender violence laws a condition for its support for a Popular Party-Ciudadanos coalition government in the region, prompting an outcry. It eventually dropped the demand.
“Denying gender violence and the need for policies of equality is a radical position which is outside of democratic norms,” Deputy Prime Minister Carmen Calvo said Tuesday.
Vox's leader in Andalusia, Francisco Serrano, sparked new controversy on Tuesday by calling the feminist mobilisation “kale borroka” — a reference to street violence by youths who supported the armed Basque separatist group ETA in Spain's northern Basque Country.
Vox's performance last month in Andalusia was the first time a far-right party has won representation in a Spanish regional parliament since the country returned to democracy following the death of longtime dictator
Francisco Franco in 1975.
It has reshuffled the political cards in Spain with polls showing that Vox along with the PP and Ciudadanos could win a majority at the national level as well.
The protests against Vox come less than two months before feminist groups in Spain mark International Women's Day on March 8 with another strike.
Last year over five million men and women took part in an unprecedented two-hour strike held on International Women's Day.
Before Vox's rise, the fight against gender violence enjoyed wide cross-party support in Spain.
The Spanish parliament in 2004 unanimously passed Europe's first law specifically cracking down on gender-based violence that offered free legal aid and established special courts for victims.
Around 50 women are killed each year in Spain by their romantic partner or former partner.
Each death receives intense media coverage and is condemned by parties across the political spectrum.