In a statement, Mayor Jules Radich said he hoped the rally on Monday, November 18 would send a message to decision-makers.
"This new southern region tertiary hospital is vital to the healthcare of over 350,000 people and it has been planned at the right size to meet their needs. Any deviation from what was agreed and promised will be met with extreme disfavour."
Presenters will speak at the rally, which will also feature local entertainment.
Health Minister Shane Reti had been invited and would be listened to with courtesy if he accepted, Mr Radich said.
"We welcome savings that can be made which do not affect clinical services.
"Equally, we will not accept any reductions that compromise healthcare," he said.
The free event will be held between 6pm and 7.30pm on Monday, November 18 at the Dunedin Town Hall — further details will be published online.
Mr Radich will be overseas on the night of the rally but is expect to contribute by video. —APL