In Beni, protesters have set up blazing street barricades and attacked an Ebola isolation centre, with almost 20 patients fleeing.
Martin Fayulu, an opposition leader, shared his displeasure with the electoral commission's decision on social media.
Director-General Tedros Adhanom Ghebreyesus does not mention the election but expresses concern after protests erupted on Thursday and Friday in two Ebola-hit cities that have been barred from voting on Sunday.
Reacting to an announcement by the Independent Election Commission (CENI) that over 1.2 million people will not vote in Beni, Yumbi and Butembo as a result of conflict and the Ebola outbreak, Katumbi said the decision was unjustified. He called on all Congolese to remain calm and vote without violence.
Ambulances drive into an attacked ebola treatment center during protest by demonstrators over their exclusion from the presidential election in Beni, Democratic Republic of the Congo.
This decision was taken in spite of pleas by the Congolese Foreign Minister, Leonard She Okitundu to the European Union to at least lift the sanctions for a "probationary period", reports Global News. Some 17 had already tested negative for Ebola, while seven people had not yet been tested, according to a spokeswoman for the country's health ministry.
The World Health Organization chief warned Friday that growing unrest in eastern Democratic Republic of Congo was disrupting efforts to rein in an Ebola outbreak, threatening gains made in fighting the deadly virus.
Congo's about 40 million registered voters will decide the fate of a vast country that is rich in the mineral wealth central to smartphones and electric cars but desperately poor in infrastructure and basic services.
Even so, the vote will continue to take place in the rest of the country as scheduled on December 30, and the next president will be sworn in on January 18, it said. The latest postponement pushed the vote from December 23 to Sunday after a fire in the capital, Kinshasa, destroyed voting materials.
Voting was postponed in the opposition strongholds of Beni and Butembo in the east, and the city of Yumbi in the west.
If the elections are "free and fair", an opposition candidate will nearly certainly win, according to Jason Stearns of the Congo Research Group, based at the Center on International Cooperation at New York University.
Beni has been affected by an Ebola outbreak that has killed at least 300 people since August.
In his comments on Thursday evening, Mr Kabila claimed that Ebola could spread as people taking part in the elections had to use voting machines, which required tapping on a touchscreen to select candidates.
They are Kabila's hand-picked successor, Emmanuel Ramazani Shadary, a hardline former interior minister; Fayulu, until recently a little-known legislator and former oil executive; and Felix Tshisekedi, head of a veteran opposition party, the UDPS.
Two-thirds of DR Congo's estimated 80 million population are under the age of 25, which means that a "Kabila Generation" could have a huge say on polling day.