STORY: On May 3 a teenage boy killed nine pupils and a security guard in the first school mass shooting in Serbia, and a day later a 21-year-old man killed eight outside Belgrade.
Opposition parties, who organized the demonstration, say the government led by President Aleksandar Vucic has failed to rein in media that have promoted violence and act against criminal elements in society.
Protesters called on Vucic, Interior Minister Bratislav Gasic and the head of Serbia's security agency to resign.
Reuters estimated the protest, the third of its kind this month, drew a crowd in the tens of thousands.
Protesters gathered outside Serbia's parliament before marching through the capital and onto a highway bridge over the River Sava, bringing traffic to a halt.
The government denies the opposition parties' allegations and accuses them of organizing the protests for political gain.