Muigai says swearing-in Mr Odinga would be an illegality attracting treason charges.
Kenya's Attorney-General Githu Muigai has warned against plans to swear-in opposition National Super Alliance (Nasa) leader Raila Odinga as the ‘people’s president’ on December 12.
Prof Muigai, the chief legal adviser to the government, on Thursday said swearing-in Mr Odinga would be an illegality attracting charges of treason.
He said such an inauguration would be outside the Constitution because Kenya already has a sitting president who was inaugurated on November 28.
"I reiterate the any attempts to swear in any person as president other than the one elected in line with the constitution and in a manner provided for in the law is unlawful, illegal null and void ab initio," Prof Muigai told journalists at the State Law office in the capital Nairobi.
Nasa is planning to swear in Mr Odinga using the results of August 8 presidential election on Tuesday to coincide with Jamhuri Day celebrations although they have not disclosed the venue.
Mr Odinga, who pulled of the October 26 repeat presidential poll, has maintained that he won the August 8 poll but was rigged out through manipulation of numbers in IEBC servers.
At the same time, the AG warned the 11 counties that have passed people’s assembly motion, saying they committed illegalities.
Such devolved Houses, he said, may be surcharged for misusing taxpayers’ money.
He said the national government retains the right to review the funds allocated to the concerned counties.
The AG's warning came hours after Mr Odinga told off the US and other western powers that have advised against the oath plan.
The US, Mr Odinga said, should not talk about violation of the constitution as far as his swearing-in is concerned yet Washington DC has been "loudly silent" on killing of innocent protesters by police since August.
Kenya Police stand accused of shooting and killing dozens in violent protests that followed August 8 General Election and October 26 repeat presidential poll.
"We thought we had friends but we were wrong and now we know they are enemies. Up to today, no single ambassador has condemned the killing of people by police," he said.
"The western countries should, therefore, keep off the affairs of the country as Kenyan problems will only be solved by Kenyans themselves."
He spoke at the City Mortuary in the capital Nairobi when he met and condoled with families that lost their loved ones during the post-poll chaos.