Nigeria's government has announced that President Olusegun Obasanjo is to replace Vice-President Atiku Abubakar.
Mr Abubakar said he would take legal action over the decision
A government spokesman said Mr Abubakar had "technically resigned" by attending an opposition convention.
The ruling People's Democratic Party expelled the vice-president from its ranks on Friday and called for him to be dismissed from his government post.
The two leaders fell out early in the year, when Mr Abubakar opposed plans to let the president run for a third term.
Presidential spokesman Uba Sani said President Obasanjo, who must step down after elections in April, would abide by the constitution, and called on Nigerians to remain calm.
He said the decision followed Mr Abubakar's defection to the opposition Action Congress (AC) early this week.
During the convention, Mr Abubakar was chosen as the party's presidential candidate for leadership elections next April.
The vice-president said the government's decision was against the country's constitution.
"The 1999 constitution, which is the ground norm of our democracy, does not give the president power to either declare the office of the vice-president vacant or to withdraw the rights and privileges of the vice-president," he said.
He said he would take legal action "to challenge this illegality".
But the ruling party said constitutional rules stated that the vice-president must belong to the same party as the president.
The PDP suspended Mr Abubakar last September after he was indicted on corruption charges by the Economic and Financial Crimes Commission.
Mr Abubakar has been President Olusegun Obasanjo's deputy since May 1999.
Under the country's constitution, Mr Obasanjo can only serve two terms of eight years as president.