By Aleksandar Vasovic

BELGRADE Tens of thousands of Serbians rallied in the capital Belgrade on Monday, demanding better security, a ban on violent content on TV, and the resignation of key ministers, days after two separate mass shootings.

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 last respects, to do our best so this never happens again, anywhere," said Borivoje Plecevic from Belgrade.

Last Wednesday, a schoolboy who brought two handguns to his schoo, killed eight pupils and a security guard. Six other pupils and a teacher were wounded.

Two days later, a 21 years-old man brandishing an assault rifle and a pistol killed eight and wounded 14 people in central Serbia.

Both shooters surrendered to the police.

Protesters and opposition supporters demanded a shutdown of TV stations and tabloids they accuse of promoting violent and vulgar content.

Opposition parties and some rights groups accuse President Aleksandar Vucic and his ruling Serbian Progressive Party (SNS) of autocracy, oppressing media freedoms, violence against political opponents, cronyism, corruption, and ties with organised crime. Vucic and his allies deny this.

Protesters also called for the resignations of Interior Minister Bratislav Gasic and Aleksandar Vulin, the director of the state security agency, and the dismissal of the government's Regulatory Committee for Electronic Media (REM) within a week.

Education minister Branko Ruzic resigned on Sunday.

Demonstrators also demanded an emergency parliamentary session and a debate about the overall security situation.

"(This is an act of) solidarity against ... violence in media, in the parliament, in everyday life ... solidarity because of lost children," 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 over 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.

(This story has been corrected to change the number of victims in the school shooting to eight pupils and a security guard in paragraph 4)

(Reporting by Aleksandar Vasovic, Fedja Grulovic and Branko Filipovic; Editing by Mark Potter)