Although an invitation had been sent to his office, Najib was not present when the event started at 3.30pm.
Mahathir, 92, was alone on stage to address questions from the audience of about 500 on contentious issues that have been raised about both the Pakatan Harapan chairman and the prime minister.
No Umno representative made an appearance, including party supreme council member Puad Zarkashi who last month offered to debate Mahathir on Najib’s behalf.
Mahathir, who was prime minister from 1981 to 2003, also commented on Najib’s absence, saying the prime minister “may have another programme” to attend.
“With Najib’s absence at this forum, it is clear he has a lot to hide.”
Mahathir had arrived with his wife, Dr Siti Hasmah Mohamad Ali, at the Dewan Raja Muda Musa here.
He had last month requested Najib to join him at the forum in answering allegations and slander hurled at them.
Other Pakatan Harapan leaders present included Khalid Abdul Samad, Daroyah Alwi, Mukhriz Mahathir, Muhyiddin Yassin, Zuraida Kamaruddin, Ng Sue Lim, Lim Guan Eng, Lim Kit Siang and Zaid Ibrahim.
On July 25, a group of PPBM Youth members arrived at the Putra World Trade Centre to deliver the formal invitation for Najib to attend the forum organised by the wing.
An earlier “Nothing2Hide” forum was arranged in June 2015. Najib had initially agreed to make an appearance but did not turn up after police advised him against it, citing security concerns.
Mahathir, who came to that forum unannounced, went on the stage to address the audience of about 2,000 people. He spoke for only six minutes before he was escorted off the stage by police