A second referendum on the document is scheduled for Friday, but it is widely expected to be adopted.
"Before the decisive day of the referendum in Egypt activists came to the Embassy of Egypt in Stockholm to support Egyptian heroes who are resisting the sharia-dictatorial draft of the constitution of the president Morsi," Femen said in a statement published on its website.
Holding a Koran in front of her privates, al-Mahdy was joined by two other women outside the embassy on the posh waterfront avenue Strandvägen in central Stockholm.
Body painting scrawled across the women's chests read "Sharia is not a constitution" and linked the apocalypse to Egyptian president, who represents the Muslim Brotherhood party, citing fears that the constitution would impede Egypt's democratic development.
Femen's website supported al-Mahdy's action, stating in an expletive-ridden text that Morsi's "last resting will be the Nile with crocodiles, not the pyramids" if he started shooting at his own people.
The nude guerrilla tactics of Femen originally started in Ukraine and the "striptivist" demonstrations have since spread.
In October, a group from Femen staged a nude protest at a German outlet of Swedish furniture giant Ikea in reaction to the company's decision to remove women from the version of its catalogue distributed in Saudi Arabia.