Serbia's public prosecutor has indicted 13 people including a former minister over the collapse of a concrete canopy that killed 15 last month and triggered weeks of massive anti-government protests.

Prosecutors arrested 13 people over the tragedy, but much of the public is skeptical about the honesty of the investigation

Serbian university students left piles of old school books outside the education ministry building on Friday as part of almost daily street protests demanding accountability over the collapse nearly two months ago of a concrete canopy that killed 15 people in the country's north.

Separately, Serbia’s government extended school winter holidays by starting them nearly a week earlier as its continues to grapple with widening student protests challenging the rule of populist President Aleksandar Vucic. The move is seen as a government ploy to curtail protests.