Mr Ralston said only a public movement in support of 1930s social justice campaigner and commercial radio pioneer Colin "Uncle Scrim" Scrimgeour compared to the support voiced for Mr Campbell.
"You have to go back a long way in history to find this kind of public response."
Rallies will happen in Dunedin, Christchurch, Wellington and Auckland.
"The response has been overwhelming," Auckland Save Campbell Live rally organiser Ruth Crichton said this afternoon. "I wasn't sure what to expect."
She said New Zealanders would miss out on an important source of information if Campbell Live was axed.
Ms Crichton, a student, said she was constantly receiving messages and phone calls from people who wanted to support the current affairs show.
The Save Campbell Live Facebook group, Action Station, Scoop Media and the Coalition for Better Broadcasting supported the rallies.
The current affairs show was often cited as being locked in a ratings war with TV One's Seven Sharp but a senior media studies lecturer said the threat to drop Campbell Live epitomised "a deeper structural malaise" in New Zealand's media sector.
"MediaWorks' concern is not primarily Campbell Live's rating lower than Seven Sharp but its marginal returns and capacity to deliver audience flow to subsequent programmes," Victoria University's Dr Peter Thompson wrote in The New Zealand Herald this week.
"Even if Campbell Live were out-rating Seven Sharp, the relentless drive to maximise returns from every slot could make it economically rational to replace it with a programme that is cheaper or more popular," Dr Thompson said.
"...If commercial logic is allowed to drive the entire television sector, we will likely be left without any regular, substantive current affairs in prime time. That would be a tragedy for democracy."
MediaWorks bosses reportedly told Campbell Live staffers the current affairs programme could be dumped for a daily entertainment show.
Yet MediaWorks has denied the Jono and Ben show was considered as a replacement for Campbell Live.
Mr Campbell said he wasn't able to comment at this point in time.
MediaWorks did not immediately respond to a message for comment.
The Auckland rally on Friday would start on the corner of Karangahape Rd and Symonds St at 11.30am. Those in attendance planned to march to TV3 owner MediaWorks' headquarters in Flower St, Eden Terrace. There, Campbell Live supporters planned to present a petition to the broadcaster.
The Wellington rally would start at 4.30pm at Te Aro Park, with a noon rally in Christchurch at Hagley Park, and the Dunedin rally in the Lower Octagon at 4.30pm.
