Kenya's main opposition has called three days of nationwide rallies next week to protest at the 27 December polls it says were effectively stolen.
The ODM's Anyang N'yongo (L) and Raila Odinga spoke to reporters
Defeated presidential candidate Raila Odinga and his Orange Democratic Movement plan protests from Wednesday to Friday, in 25 towns and cities.
Accusing the government of showing no interest in dialogue or mediation, the party also called for sanctions.
Police immediately announced they would not allow the rallies to go ahead.
National police chief Maj-Gen Mohamed Hussein Ali told reporters such protests were "not appropriate at this time".
Previous attempts to march through Nairobi were blocked by riot police with water cannon and tear gas.
Protesters were kept hemmed into city slums like Kibera, unable to reach the symbolic Uhuru (Freedom) Park rallying-point, which Mr Odinga has chosen again as the focus for the protests on Wednesday.
The park has been surrounded by a ring of steel for the past two weeks, the BBC's Karen Allen reports from Nairobi.
Some 600 people have died in violent unrest since the announcement of Mwai Kibaki's re-election as president at the end of December.
A mediation bid by Ghanaian President John Kufuor, who heads the African Union, has failed while Kofi Annan, the former UN secretary general, is due to make his own bid early next week.
Giving details of his party's plans, ODM Secretary General Anyang N'yongo called for international sanctions to be slapped on President Kibaki.
World leaders would be irresponsible to trust him with "a single cent", he said.
Mr N'yongo also denied that the new protests were pre-empting Mr Annan's visit, and said leaders did not fear arrest as they had been "arrested before".
A government minister condemned the opposition's plans for further action, saying "dialogue is not engaged in the streets".
"Dialogue suggests that people resolve their differences peacefully, over a table, not through destroying property and killing innocent Kenyans," Local Government Minister Uhuru Kenyatta told reporters.
Samuel Kivuitu, head of the Kenyan electoral commission, has disowned presidential election results published in the media on Thursday.
The results matched those he announced on 30 December and were given to the media by election officials but Mr Kivuitu said "outside forces" were behind their publication now.
He has called for an independent inquiry into allegations of fraud and says he cannot be sure that Mr Kibaki won the election.
Top US Africa official Jendayi Frazer and four African ex-presidents have been in Kenya trying to end the crisis in what had been seen as a relative beacon of stability in East Africa.
Mr Odinga's Nairobi power base is among the poor of the slums
Our correspondent says Mr Annan will have more time to spend in Kenya than Mr Kufuor, who stayed for just two days.
She says there are hopes that such a high-profile figure may be able to make progress.
European Union observers said they had concerns about the way the votes were added up, which could have affected the official results.
But Mr Kibaki has sworn in most of his key ministers and says the election is "finished".
Minor ministerial posts have not been filled, leaving the way open for a possible government of national unity.
Mr Odinga, however, has refused to join a power-sharing administration led by Mr Kibaki.
The opposition leader rejected an offer to meet Mr Kibaki, unless it was led by mediators - a condition rejected by the president.
As well as the deaths, some 250,000 people have fled their homes, following clashes between rival political and ethnic groups.