Regional officials, who have branded the succession a coup, said the election pledge did not meet their demands that Togo revert to its original constitution, which was amended after Gnassingbe's nomination to legitimise the move.
In Togo's capital Lome, at least 10,000 people marched through the opposition stronghold of Be, waving branches and placards and demanding Gnassingbe step down in the largest protest since the 39-year-old took over.
Nigeria's President Olusegun Obasanjo, current head of the African Union (AU), told a visiting Togolese delegation that he would not endorse the decision to leave Gnassingbe as president until elections, said Obasanjo's spokeswoman Remi Oyo. She said the Economic Community of West African States (ECOWAS) would impose "full sanctions" on Togo, adding this would include travel sanctions.
The head of the AU Commission Alpha Oumar Konare said in a statement that Gnassingbe's decision to remain in power was "in violation of the Togolese constitution."
He called on Togo to take the measures expected of it by the AU, ECOWAS and the international community. "Any other step would only complicate the situation in Togo," he said.
Togo's original constitution said the head of the national assembly should take over on Eyadema's death, pending polls.
Saturday's protest dispersed peacefully after a rally but Be was tense, with groups of youths roaming the side streets and police trucks patrolling. Four demonstrators were killed in clashes in the same neighbourhood a week ago.
"We have no weapons. We only have popular mobilisation. We rely on it to push back the regime and make it leave," said Jean-Pierre Fabre, secretary-general of the main UFC opposition party, which was among the protest's organisers.
Demonstrators chanted "Eyadema son: thief" and "Chirac thief," referring to French President Jacques Chirac who had a close relationship with Eyadema. Former colonial ruler France has hundreds of soldiers permanently based in Lome.