Declared “a new Arab Spring” by Femen activists, it called for women to join topless rallies at worldwide Tunisian embassies.
In a statement published on Facebook, Femen touted the event as a cry against the “lethal hatred of Islamists – inhuman beasts for whom killing a woman is more natural than recognising her right to do as she pleases with her own body.”
Scroll down for pictures from Thursday's topless jihad day
It added: “Long live the topless jihad against infidels! Our tits are deadlier than your stones!”
It came after 19-year-old Amina Tyler posted images of herself with the words “Fuck your morals” written across her chest to the Femen-Tunisia Facebook page, earning calls for her death from a preacher who feared her act “could bring about an epidemic”.
On Thursday, images of protests underway in Berlin, Milan, Kiev, Paris, Brussels and Melbourne were swiftly posted online as the movement gained pace.
Meanwhile the whereabouts of Amina are unclear. Some say she is in a psychiatric hospital. Some have expressed fears for her life. Attorney Bochra Bel Haj Hmida insists she has spoken to Amina and that she is home with her family and well.
Yet last month Femen leader Inna Shevchenko told the Huffington Post UK that since abruptly losing contact with the young activist, she continues to fear for Amina’s safety.
She said: “We still didn't hear [the] voice of Amina and didn't see her face. Until the moment we hear word of Amina about her safety we are searching for her and continuing our international bare breasts support.”
In what is believed to be her final interview before her disappearance, Amina told Federica Tourn she believes she will be beaten or raped if the Tunisian police find her.
But she insists she is not afraid: “No, nothing they could do would be worse than what already happens here to women, the way women are forced to live every day.
"Ever since we are small they tell us to be calm, to behave well, to dress a certain way, everything to find a husband. We must also study to be able to marry, because young guys today want a woman who works.”
Other groups and individuals have also declared April 4 an international day of action.
More than 107,000 activists have signed a petition calling for those who have threatened Amina’s life to be prosecuted.
An open letter calling for an International Day to Defend Amina has been signed by many feminist and atheist activists, including Richard Dawkins.
The letter says: "On the day and beyond, groups and individuals can join in by highlighting her case, posting topless photos of themselves and their activism on social media sites, signing a petition, Tweeting #Amina, writing letters in her defence, and more.
Femen said in a statement they were furious about the "barbarian threats of the Islamists about the necessity of reprisals against the Tunisian activist Amina,
"We are afraid for her life and we call on women to fight for their freedom against religious atrocities.
"Use your body as a poster for the slogans of freedom. Bare breasts against Islamism."
Femen was founded in 2008 in Ukraine. It claims to be active in 17 countries and to have more than 150,000 supporters.