What started out as a peaceful assembly turned chaotic when demonstrators breached police barricades to enter the out-of-bounds Dataran Merdeka.
Following the infraction at 2.50pm, police fired water cannons and several rounds of tear gas to disperse the large crowd that had gathered in front of Jalan Raja and Jalan Parlimen.
City police chief Deputy Comm Datuk Mohmad Salleh said they had no choice but to take stern action because the crowd had started throwing objects and charging towards Dataran Merdeka.
Earlier in the day, the gathering had a festive air to it, with the demonstrators exchanging friendly greetings with the police and some even presenting flowers to them.
Bersih co-chairman Datuk S. Ambiga had promised that the event would not be political, but that was not the tone it assumed when supporters of political parties began shouting "reformasi". Opposition leaders addressing the crowd did not help matters.
Bersih steering committee member Wong Chin Huat was quoted in a news portal as admitting that the organisers "lost control" of the thousands of rally participants who ignored instructions not to breach the barricades.
Ambiga said at a press conference that Bersih did not support the action of demonstrators who breached the barricades and called on the police to investigate.
"They should face the full brunt of the law and Bersih will fully cooperate in the investigation," she said.
Ambiga also said Bersih 3.0 organisers had issued guidelines that no one should utter anti-party or pro-party slogans.
It is believed that the crowd became unruly after a speech by PKR deputy president Azmin Ali who asked the demonstrators "if you want to occupy Dataran Merdeka". His question was met with cries of "hancur rempuh, buka pagar" (crash and destroy, open the gates).
"The Dataran does not belong to Kuala Lumpur City Hall. It belongs to the people," he told the cheering crowd at the intersection of Jalan Raja and Jalan Tun Perak.
Some demonstrators had tried to breach the barriers earlier when they trampled on barbed wire and pushed back the metal barriers but Bar Council observers pacified them.
By 2pm, the demonstrators had made their way to several streets surrounding the square.
Earlier at Masjid Jamek, Ambiga said that Bersih had achieved its goal of calling for clean, free and fair elections.
"We have also called for an independent Election Commission and respect for the rakyat's constitutional rights to live in a healthy environment," she told the crowd.
Ambiga then called on the demonstrators to disperse and was joined by Opposition Leader Datuk Seri Anwar Ibrahim on a truck moving towards Dataran Merdeka.
As Anwar reached Jalan Raja and was about to address the crowd, a large group breached the barricades and rushed towards the square.
Bernama reported that Anwar earlier urged demonstrators at the National Mosque to "proceed to Dataran Merdeka whatever happens", adding that they should not become dispirited or fearful of being arrested.