National Super Alliance (Nasa) leader Raila Odinga now says, like President Uhuru Kenyatta, he will also be sworn in as "the people’s president".
He made the declaration on Tuesday moments before police used teargas to stop him and his supporters from marching to Jacaranda grounds in Embakasi, Nairobi, for a prayer rally.
Mr Odinga said he will take oath of office on December 12 using Chapter 1 of the Constitution, which states that sovereign power belongs to the people.
"I am not a coward, I will be sworn as president on Jamhuri Day, I am the legitimate president," he told his supporters, who defied police operation to listen to him, during a stop-over on Mayanja Road.
The swearing-in, he said, will be based on the results of August 8 presidential election, which he claims he won but was rigged out through manipulation of figures.
“Server indicated that we had 8.8 million votes while Kenyatta had 7.1million votes,” he said.
He reiterated his vow not to recognise President Kenyatta and his government despite being sworn in on Tuesday.
“We went to the courts and they said Kenyatta was not duly elected… They ordered IEBC to open servers but they refused because they knew all the secrets were there.”
The inauguration, Mr Odinga, will be like that of new Zimbabwe President Emmerson Mnangagwa and not Kizza Besigye of Uganda.
Mr Besigye, President Yoweri Museveni’s doctor-and-friend-turned-foe, has become the symbol of police repression and brutality in Uganda.
“Our first resolution is that we don't recognise Kenyatta as president,” Mr Odinga said.
Clashes between Mr Odinga's supporters started in the morning, with officers in anti-riot gear sealing off the venue as early as 8.30am.
The opposition leaders said they wanted to mourn and pray for their 54 supporters they claim were killed by police in the aftermath of the October 26 repeat presidential election.
Nairobi County Police Commander Japhet Koome had warned the opposition against holding the meeting.
On Sunday, Mr Koome denied knowledge of the Nasa event and, a day later, vowed not to allow it, saying the only major public event that would take place in Nairobi on Tuesday would be President Kenyatta's swearing-in.
But Nasa co-principal Moses Wetang'ula on Monday dismissed Mr Koome's claims.
“We are planning prayers for our supporters who were shot dead during protests. We will also fundraise for the victims’ families so that they can hold a befitting send-off for their loved ones,” Mr Wetang'ula said.
The Jacaranda meeting was a culmination of a series of memorial services held in other parts of the country.
Mr Odinga, who pulled out of the October 26 repeat presidential election, has vowed not to recognise Mr Kenyatta’s presidency despite the Supreme Court upholding the Jubilee leader's 98 percent-of-votes-cast win.
All Nasa leaders turned down President Kenyatta’s swearing-in invitation, with Mr Odinga terming it "a feast" as other Kenyans "mourn".
“We are mourning, and on the other side, Uhuru is preparing a feast, a ceremony. All patriotic Kenyans will be at Jacaranda mourning our people brutally killed by this regime,” said Mr Odinga.