Kenya's opposition has said it will challenge the results of last week's presidential election in the supreme court and wage a campaign of "civil disobedience".
Opposition leader Raila Odinga told reporters that Kenyans will not willingly go along with "democracy's slaughter".
Mr Odinga has claimed that election results which returned President Uhuru Kenyatta to office were hacked and rigged.
The election commission has said there was a hacking attempt, but it failed.
The opposition last week said going to court was not an option, but Mr Odinga's remarks contradict this.
His challenge after losing the 2013 election was unsuccessful.
Mr Odinga's comments have the potential to set off another wave of protests in the capital, Nairobi, and elsewhere, which have already led to at least two dozen people being shot dead by police since the August 8 vote.
"We will not accept and move on," he said.
"We shall hold vigils, moments of silence, beat drums and do everything else to draw attention to the gross electoral injustices."
Streets of Nairobi's populous slum of Kibera were empty ahead of Mr Odinga's statements, with businesses closed as they awaited his announcement.
The opposition stronghold has seen some of the worst clashes between police and civilians protesting against the results after the electoral commission announced Mr Kenyatta's win late on Friday.
Meanwhile, Kenya's interior minister Fred Matiangi stopped an attempt to de-register two civil society groups that pointed out anomalies in the presidential election.
The National Non-governmental Organisations Board has said it de-registered the Kenya Human Rights Commission and called for the arrest of the directors of the Africa Centre for Open Governance.
Both groups monitored the elections.
In a statement, Mr Matiangi said a committee will be formed to look into allegations that the two groups had tax compliance issues and one was not registered.
He also ordered that no action be taken against the groups for 90 days.