Police said they suspected the woman Serata's divorced husband of barging into her house in the capital of Ghazni province and murdering her, alongside their eight-year-old son and nine-year-old daughter.
"The children saw the killer take their mother's head off, so he killed them too," a local policeman told Reuters, adding that the attacker had spared Sereta's two-year-old daughter.
Activists say there has been a sharp rise in violent attacks on women in Afghanistan over the past year.
They blame President Hamid Karzai's waning attention to women's rights as his government prepares for the exit of most foreign troops in 2014 and seeks to negotiate with the Taliban, Afghanistan's former Islamist rulers.
Excluding Serata's beheading, there have been 16 cases of "honour killings" recorded across the country over March and April, the first two months of the Afghan New Year, according to Afghanistan's Independent Human Rights Commission (AIHRC).
This compares to the 20 cases recorded for all of last year, said commissioner Suraya Subhrang, blaming increased insecurity and weak rule of law for the sharp rise. Since AIHRC started recording such killings in 2001, there have never been more than 20 cases a year.
"And there are many that go unreported. Men make a quick decision in their own courts to kill a girl and hold a prayer for her the next day," Subhrang told Reuters.
Serata divorced her husband Mohammad Arif, 38, a year ago after enduring almost a decade of domestic abuse, said Shukria Wali, head of Ghazni's department of women's affairs, which is attached to the ministry in Kabul.
Police said they were still hunting for Arif. Violent crimes against women often go unpunished in Afghanistan, with activists blaming police carelessness, corruption and a growing atmosphere of impunity.
Officers investigating the case described it as an honour killing - a phrase used to describe the murder of mostly women and girls by people who accuse them of besmirching a family's reputation.
Afghan women have won back basic rights in education, voting and work since the austere rule of the Taliban was toppled just over a decade ago, though fear now mounts that freedoms will be traded away as Kabul and Washington seek talks with the Islamist group to secure a peaceful end to the war.
Donors are expected to commit just under $4 billion in development aid annually to Afghanistan at a summit in Tokyo on Sunday, though the European Union - the biggest contributor - has said maintaining its support will be difficult if women's rights are not protected.
During its five-year reign, the Taliban banned women from most work, denied them the right to vote and ordered that they wear a head-to-toe burqa when out of the house.
Despite more than 10 years of war and billions of dollars in foreign aid, the country remains desperately poor. Female illiteracy rates in rural areas are above 90 percent and child marriages are still widespread despite being illegal.
© Thomson Reuters 2012