While it was unclear who was behind the fracas, it highlights that Mahathir - Malaysia's prime minister for 22 years before stepping down in 2003 - is seen as a political threat.
A mass brawl was sparked off after security members were forced to retaliate.
Tun Dr Mahathir Mohammad giving a speech during the Nothing 2 Hide 2.0 Forum at Raja Muda Musa Hall Shah Alam August 13, 2017. A Parti Pribumi Bersatu Malaysia (PPBM) leader said today the party proceeded to hold the "Nothing to Hide 2.0" forum, despite being tipped off about potential trouble as its chairman Tun Dr Mahathir Mohamad was not afraid to confront his critics.
"We received intel that they were planning to cause problems", the party worker said. "So we will call up the organisers for questioning", he added.
Most of the suspects appeared to be youths in their early 20s.
He said that if people were to choose Datuk Seri Anwar Ibrahim as the prime minister, there is no reason for him to reject their choice. "How can you justify (the force)?" the man asked with a hostile tone.
Dr Mahathir, responding calmly, said, "Four policemen were killed".
Just as the audience expected the forum to resume with the next person in line ready to take action, a commotion broke out as one assailant threw his shoe in the direction of Dr Mahathir.
They also set off flares during the short outburst of violence in the community hall where the event took place, with video footage showing the room filling with smoke.
PPBM's security workers eventually managed to repel the group of troublemakers to the exit and later apprehended three suspects. Forum organizer Syed Saddiq Syed Abdul Rahman said several people were injured but couldn't give details on the exact number or their condition.
"There was definitely an element of sabotage and also an attempt to hurt Tun Dr Mahathir because... the bottles, shoes and later flares were thrown not to the sides or back, but to the front towards Tun Dr Mahathir", he said.