Crowds in numbers not seen in the Balkan country for years, solemnly marched through the city centre behind a banner reading "Serbia Against Violence."
"We have gathered here to pay our final respects to the murdered children and to take every possible action to prevent such atrocities from ever happening again, anywhere," said Borivoje Plecevic from Belgrade.
Last Wednesday, a schoolboy who brought two handguns to his school, killed six pupils, a teacher and a security guard. Six other pupils and a teacher were wounded.
A day later, a 21-year-old man brandishing an assault rifle and a pistol killed eight and wounded 14 people in central Serbia.
Both shooters are under arrest.
Protesters and opposition supporters demanded a shutdown of TV stations and tabloid newspapers they accuse of promoting violent and vulgar content.
Demonstrators also demanded an emergency parliamentary session and a debate about the overall security situation.
"I am here to demonstrate solidarity against the pervasive violence in the media, in Parliament, and daily life," said Snezana, a woman in her 60s who declined to give her last name.
Similar protests were held in several other Serbian cities.
In response to the shootings, Serbia's police on Monday started a one-month amnesty for surrendering illegal weapons. It said more than 1,500 were handed over on the first day.
In addition to existing gun laws, Vucic announced police checks of registered gun owners.
Serbia has a deeply entrenched gun culture, and along with the rest of the Western Balkans is awash with military-grade weapons and ordnance in private hands after the wars of the 1990s that tore apart the former Yugoslavia.
Reuters