About 10,000 people joined in the first rally in Yugoslavia since the end of NATO bombing yesterday to demand democratic change and call for President Slobodan Milosevic to stand down.
Mr Vladan Batic, leader of the Christian Democrats and a co-ordinator of the Alliance for Change - the umbrella opposition group that organised the technically illegal rally - listed demands he said would be repeated across Serbia this summer. They included the resignation of Mr Milosevic and early elections.
Mr Batic told the crowd: "We'll go from town to town, from house to house, from man to man, and light the torch of democracy in Serbia."
An explosion went off in Cacak's central square just as the demonstration got under way. No one was injured by the device, normally used in military exercises to simulate artillery blasts.
The crowd started chanting "Red Bandits, Red Bandits" after the explosion. In Serbia that cry normally refers to Mr Milosevic and his supporters.
Earlier, organisers told the protesters they had been informed by police that the rally was not authorised but pledged that they would go ahead with it anyway.
Although Yugoslavia has ended the state of war, a ban on public gatherings has not yet been lifted since it came under a Serbian rather than a federal government decree.
Thousands of supporters attended, despite efforts by police and authorities to stop them from arriving in Cacak. Police stopped two buses and 10 cars from Belgrade carrying supporters, party officials and foreign and local journalists en route to Cacak. Reporters said they were turned back.
Organisers said police had also stopped other buses carrying supporters from central Serbian towns. Some walked to Cacak while others were picked up by local Yugoslav army soldiers and taken there, they added.
During his speech, Mr Vuk Obradovic, leader of the Social Democracy party, thanked the army for its assistance.
"Thanks to our army for enabling people to break through the police barricades on some roads and come to join us. They are with us and we are with them and they know it," he said.
The area around Cacak witnessed several days of protests last week by army reservists who blocked roads and bridges and demanded wages for time served in Kosovo during NATO air strikes. It was not clear if reservists had been the ones ferrying the supporters to the rally.
The Democratic Party said most state bus companies had cancelled their services from central Serbian towns to Cacak, about 150 km south of Belgrade, though this did not stop thousands from attending.
The crowd applauded wildly when the mayor of Cacak, Mr Velimir "Velja" Ilic, a member of an opposition party, appeared on stage. Mr Ilic went into hiding a month ago after criminal charges were brought against him for organising a rally during the state of war.
He told the crowd in his first public appearance since May: "Today I am the happiest person in the world because I am with you again. But I could cry because of what this government has done to this country."
The Alliance for Change plans rallies over the summer in 20 other cities and towns, and has begun circulating a petition calling for Mr Milosevic to step down.
British soldiers shot dead an armed man in the town of Lipljan yesterday, a British army spokesman said.
Lieut Col Robin Hodges said three soldiers in the town, 15 km south of Pristina, had fired in self-defence after the man came round the corner of a building brandishing a pistol.