Kenyan opposition leaders say they will continue to hold rallies demanding new elections, despite a government ban.
The opposition has called for a fresh round of elections
Kenya outlawed opposition rallies and dismissed calls for elections, after voters rejected a new constitution.
Vice-President Moody Awori said demonstrations supporting calls for elections would be seen as a "threat to national security".
Opposition leader Raila Odinga said it is unconstitutional to deny people freedom of assembly.
Mr Odinga headed the "No" campaign in the constitutional referendum a week ago, arguing that the new constitution proposed by the government concentrated power in the hands of the president.
"This is a government in panic and it wants to run away from the people" William Ruto, secretary general of the opposition Kanu party, told the AFP news agency.
President Mwai Kibaki's defeat in the referendum prompted calls for new elections.
Opposition parties have called for a series of nationwide rallies, starting on 10 December.
The government declared a ban on pro-election rallies following Saturday's "No" campaign victory rally in the capital, Nairobi.
Mr Awori said in a statement that opposition calls for nationwide protests were "inappropriate".
He said the rejection of the constitutional reforms was not grounds for new elections and said the French and Dutch governments had not resigned after losing referendums this year.
Many Kenyans used the vote to protest against Mr Kibaki
"The government considers these calls for nationwide rallies inappropriate and a threat to national security," he said.
"Accordingly, the government will not allow the planned rallies and [citizens] are cautioned not to attend the meetings," he said.
Tens of thousands of people attended the opposition rally in Nairobi.
Correspondents say many Kenyans used the referendum to protest against what they see as Mr Kibaki's poor performance since he was elected three years ago.
The president, who had led the "Yes" campaign, dismissed his entire cabinet in the wake of the referendum defeat.
The issue divided the cabinet - Mr Odinga was one of seven ministers who joined the "No" campaign.
He was a key figure in Mr Kibaki's 2002 election victory but the pair fell out over the question of the draft constitution.
Some campaigning during the referendum was marred by violence.
But by banning such expressions of dissatisfaction, the BBC's Adam Mynott in Nairobi says the fear is that Mr Kibaki will further alienate large sections of the population.