Such an inauguration would worsen the rifts opened by an acrimonious election season, when more than 70 people died in political violence.
"The criminal law of the Republic of Kenya stipulates that sort of process is high treason".
Under Kenyan law, treason is punishable by death. "It is high treason of the persons involved, and any other person facilitating that process", he told a news conference.
Muigai said people's assemblies proposed by the National Super Alliance, Odinga's opposition coalition, were illegal as well. "Parliament has not delegated any such power and the effect for that is that those institutions are unconstitutional, illegal and are null and void", said the Attorney General.
At the same time, the AG warned the 11 counties that passed the people's assembly motion, saying they have committed illegalities.
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Kenya's opposition leader is rejecting U.S. advice to cancel a planned mock inauguration of him as president, saying he doesn't recognize the legitimacy of President Uhuru Kenyatta.
Mr Odinga successfully challenged Mr Kenyatta's August re-election but then boycotted a rerun vote in October, demanding electoral reforms.
Prof Muigai, the chief legal adviser to the government, on Thursday said swearing-in Raila Odinga would be an illegality attracting charges of treason.
The AG's warning came hours after Odinga told the United States and other western powers that have advised against the planned swearing-in plan to back off.
"We thought we had friends but we were wrong and now we know they are enemies".