Protesters, holding banners with photos of the victims -- who include a nine-year-old girl -- rallied outside the Presidential Palace.
Holding up the green-draped coffins, they chanted "Death to the Taliban" and called on President Ashraf Ghani and Chief Executive Abdullah Abdullah to resign.
The four men, two women and child were found partially beheaded on Saturday in the southeastern province of Zabul, officials have said.
In the past five days, rival Taliban groups have been fighting each other in the region where the bodies were found.
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Ghani sent a delegation to Ghazni to investigate the killings and attend funerals, his office said. A statement described the kidnappers as "mainly non-local terrorists."
Civil society activist Zahra Sepehr, one of the protest organizers, estimated a turnout of about 10,000, which would make it the biggest demonstration in Kabul since the killing of a young woman, Farkhunda, by a mob in March.
"We want justice and we want this government, Ghani and Abdullah, to go so that we can have a government that protects all the people of the country and brings security to the whole country," Sepehr told The Associated Press.
She said the deaths of the seven were evidence of the lack of security across Afghanistan. This year, the Taliban has extended its reach across Afghanistan in its fight to topple the government, and the Islamic State group is also believed to have a presence in Zabul, as well as in the southeastern Nangarhar province.