The Nasa coalition is planning to swear in Raila Odinga using the results of the August 8 presidential election on Tuesday to coincide with Republic (Jamhuri) Day celebrations although they have not disclosed the venue.
He added: "Any person acting in violation of Article 3 (2) commits a treasonable offense under Section 40 of the penal code".
"The United States also urges Opposition leaders to work within Kenya's laws to pursue the reforms they seek and to avoid extra constitutional actions such as the proposed "inauguration ceremony" on December 12", the USA embassy said in a statement.
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.
Police have disputed Nasa tallies in the past, saying only a few dozen people died in clashes that rocked Nairobi and several counties in western Kenya.
Kenya's main opposition group says police have detained one of its advisers ahead of a planned protest ceremony to stage a mock inauguration of Raila Odinga as president. "Parliament has not delegated any such power and the outcome for that is that those institutions are unconstitutional, illegal and are null and void", said the Attorney General.
Without directly mentioning the ceremony or the United States, Odinga said on Thursday that Kenyans should be left to solve their own problems.
An October re-run was boycotted by Odinga and his supporters as they called for electoral reform. "Yet they have the audacity to come and advise us to forget and move on". Kenyatta won with over 98 percent of the votes and was inaugurated last week. "Which constitution, my foot", Odinga told reporters in Nairobi.
"We thought we had friends but we were wrong".