There will be no elections in 2017 if IEBC commissioners don't leave office, opposition leaders Raila Odinga and Moses Wetang'ula have said.
Addressing thousands of their supporters at Uhuru Park, the two leaders said they would name their representatives on Thursday, who are expected to join their counterparts from the government side to start talks on the IEBC stalemate.
Mr Wetang'ula, the Bungoma senator, led the crowd in a chorus calling for the commission to be disbanded, insisting the Opposition would not relent on its resolve to see the commissioners sent home.
Wiper leader Kalonzo Musyoka addressed the crowd by phone from China, where he is touring, saying their resolve was unstoppable.
Mr Odinga said the fight for a lean IEBC was on course, explaining that he was involved in the creation of the current commission but they had not envisioned the commissioners would turn out to be corrupt.
He acknowledged that he was personally involved but added that times had changed and now he wants the commissioners shown the door.
Earlier, Cord supporters started streaming into Uhuru Park in their hundreds, carrying tags with pictures of opposition chief Raila Odinga, whistling their way into the venue.
A majority of them were dressed in Orange Democratic Movement colours, with ingenious businessmen selling the whistles and tags making a killing from as early as 8am.
The crowd was enthusiastic about the rally, held parallel to the State celebrations led by President Uhuru Kenyatta at Afraha Stadium in Nakuru.
The Opposition, which has been calling for the removal of the nine electoral agency commissioners, had said on Tuesday that the rally would go on.
The defiance continued even after a meeting at State House, where President Kenyatta invited opposition leaders to the celebrations at Afraha.
On Wednesday, scores of police officers were deployed at the park, with about three lanes of frisking for every supporter who wanted access to the planned event.
About 20 police officers were placed at the gate to frisk the supporters as they streamed in.
At least 10 police vehicles from Nairobi were parked at the grounds, as the supporters went on with the cheering and whistling.
The pitch of the cheering would rise each time a helicopter flew past the park as the wait for Mr Odinga and Mr Wetang'ula continued.
Earlier indications were that Mr Odinga and his team would join President Kenyatta at Afraha Stadium in the national celebrations, the first ever to be held outside Nairobi, before flying to Uhuru Park for their rally.
Additional reporting by Obed Simiyu.