Kenya's attorney general said Thursday that an opposition plan to have its leader Raila Odinga sworn in as an alternative president would amount to treason.
Odinga challenged the result of August's presidential election but then boycotted the court-ordered October rerun, handing victory to President Uhuru Kenyatta.
The veteran opposition leader continues to dispute the legality of Kenyatta's rule and plans to have himself declared president at a ceremony on December 12, Kenya's independence day.
Attorney General Githu Muigai said any such move would be illegal.
"The criminal law in the republic of Kenya... stipulates that that process is high treason," he said, a crime punishable by death although Kenya has not executed anyone for 30 years.
"It is high treason in respect of the person so involved and any other person facilitating that process," he said.
Odinga has insisted the ceremony will go ahead, defying growing pressure to call it off amid fears it will lead to yet more violence between police and opposition supporters.
On a visit to the Kenyan capital Nairobi on Wednesday Donald Yamamoto, the acting top diplomat for Africa in the US government, urged Odinga to call off his planned swearing-in and instead "work within Kenya's laws" to seek reform.
Yamamoto said the opposition should "avoid extra-constitutional actions such as the proposed 'inauguration ceremony'."
Odinga dismissed the US pleas on Thursday. "Kenyan problems will be resolved by Kenyan people," he said. "Don't come and shout at us and tell us that we are violating the constitution."