More than 30,000 demonstrators in Belgrade yesterday heard Mr Zoran Djindjic, leader of the Democratic Party, warn: "If you touch our children, you touch our future, and this is the end. There is no step back after that."
The demand came as the Milosevic regime targeted students and youth activists from the resistance movement, Otpor, following the weekend assassination of a senior Socialist Party politician in Novi Sad.
Opposition leaders had called yesterday's demonstration to protest against a clampdown that began last week. Student activists were disappointed with the turnout. A rally last month calling for free and fair elections brought 100,000 people out in Belgrade.
The Serbian government spokesman had blamed the murder of Mr Bosko Perosevic, the head of the Socialists in Novi Sad and president of Vojvodina province parliament, on the youth movement, opposition parties and foreign governments.
Police arrested activists at the weekend, whom they released after questioning. They issued wanted posters yesterday for the arrest of two Otpor members in connection with the murder. The resistance movement said the charges were "nonsense" and that the youngsters had been in Bosnia for a month.
The regime has signalled its hard line toward Otpor, calling its members "traitors" and "fascists".
The assassination of Mr Perosevic was caught on television. He received a call on his mobile phone at an agricultural fair. He moved away from his official delegation to take the call and then was shot at close range by a security guard. Police gave chase and arrested the man.
The federal information minister, Mr Goran Matic, said the man held for the murder, Mr Milivoje Gutovic (50), was linked to the opposition. He said police found Otpor propaganda and manuals on terrorism in his flat.
But the Democratic Party said Mr Gutovic was an ardent Socialist Party disciple.