The United States has asked Kenyan opposition leader Raila Odinga to call off his planned swearing-in ceremony on Tuesday next week, Kenya's Daily Nation reported on Wednesday.
Such an inauguration would worsen the rifts opened by an acrimonious election season, when more than 70 people died in political violence. Muigai did not name anyone but opposition leader Raila Odinga said last month, he would be inaugurated by a people's assembly on December 12 - Kenya's Independence Day.
Muigai cited the Constitution which prescribes death as the punishment for treason.
"The criminal law of the Republic of Kenya stipulates that this sort of process is high treason".
Speaking at Sheria House on Thursday, the AG warned that persons that will facilitate or be involved in the event will face the full wrath of the law.
Muigai said people's assemblies proposed by the National Super Alliance, Odinga's opposition coalition, were illegal as well.
On Friday, Nasa named a seven-member People's Assembly steering committee headed by strategist David Ndii to push for its agenda. But the Supreme Court nullified the result, and a repeat election was held on October 26. Kenyatta won again, with 98 percent of the vote.
On a visit to the Kenyan capital Nairobi on Wednesday Donald Yamamoto, the acting top diplomat for Africa in the United States government, urged Odinga to call off his planned swearing-in and instead "work within Kenya's laws" to seek reform.
Donald Yamamoto, the Principal Deputy Assistant Secretary at the USA government's Bureau of African Affairs, warned that Odinga's move would further polarise Kenya.
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.
"We thought we had friends but we were wrong and now we know they are enemies. Which constitution, my foot", Odinga told reporters in Nairobi.