Attorney General Githu Muigai's comments came a day after the United States urged Odinga's supporters to call off the mock "swearing-in" on Dec. 12, the country's independence day.
The opposition objects to President Uhuru Kenyatta's re-election after months of political turmoil that included a nullified August election and a repeat vote in October, saying that electoral reforms have not been made.
The attorney general warned that institutions created by Odinga's group such as "the People's Assembly" are unconstitutional and illegal and "may be visited by the full force of the law."
Odinga on Thursday rejected the U.S. advice to cancel next week's event, saying he doesn't recognize the legitimacy of Kenyatta as president.
Odinga also repeated his accusations against Kenya security forces, saying dozens of people have been killed in the months of election unrest and "nobody is talking about it. Yet they have the audacity to come and advise us to forget and move on."
The opposition leader spoke outside a morgue in the capital, Nairobi, while accompanying families collecting bodies of alleged victims.
The U.S. Embassy statement on Wednesday called Odinga's planned inauguration "extra-constitutional." It came as the top U.S. official for African affairs, Donald Yamamoto, visited the East African economic hub and met with government officials and opposition leaders.
Odinga's National Super Alliance has said it will proceed with the event.
Odinga successfully challenged Kenyatta's August re-election but then boycotted the new vote in October, demanding electoral reforms. Kenyatta won with over 98 percent of the votes and was inaugurated last week.
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