The Supreme Court on September 1 called for a new election within 60 days after finding irregularities in the re-election of Kenyatta in last month's elections.
Kenyan president Uhuru Kenyatta says he will respect the Supreme Court's overturning of his election victory, even though he disagrees with it.
Odinga, who also contested the presidency in 2007 and 2013, repeated his statement after Friday's court ruling that the opposition would not participate in the re-run of the election without changes to the election commission.
The election commission had declared incumbent Uhuru Kenyatta the victor by a margin of 1.4 million votes.
Odinga alleged fraud shortly after counting began, claiming the system transmitting votes had been hacked, and that forms from polling stations that were meant to back up the electronic results were not being uploaded.
The credit rating agency said that a ruling to invalidate August's presidential election result was a credit negative for the East African country.
Worldwide election observers, including former Secretary of State John Kerry, had said they saw no interference with the vote. "It is indeed a very historic day for the people of Kenya, and by extension to the people of the continent of Africa".
It is believed to be the first time in Africa that a court has ruled against the electoral win of an incumbent based on a court challenge by the opposition. "Let us be people of peace".
"There will be no elections on the 17th of October until the conditions that we have spelt out in the statement are met", he said on Tuesday.
Last month's election - which included the presidential poll in addition to races at other levels of government - was one of the most expensive ever held in Africa.
Odinga's conditions for participating in the repeat presidential election include the removal of six officials at the election board.
He said the commission had committed irregularities "in the transmission of results", adding that the court would provide details in a full judgment within 21 days. "It is a whole new election with the requirements that the August 8 one had", Mr Mwangi said. "The same threats against the judiciary have been repeated at State House", said its chief Bryan Khaemba, referring to the presidential palace.
"Do you understand me?"
But while the opposition supporters were celebrating, those who voted for President Uhuru were counting their losses, perhaps bothered by the thought of returning to the polls.