Nigerians must resist the "wholescale fraud" of Saturday's presidential elections, an opposition alliance says.
Saturday's election was "a charade", said local observers
"We have seen revolutions around the world, from Ukraine to the Philippines. We must replicate that," Pat Utomi told the BBC, on behalf of 25 parties.
Outgoing President Olusegun Obasanjo has admitted the polls were flawed but says they should not be re-run.
Ruling party candidate Umaru Yar'Adua won by a landslide, according to official results.
Police were on high alert as the results were announced in the capital, Abuja.
Thousands of opposition supporters protested in the streets of the biggest northern city, Kano, but they were soon dispersed by police firing tear gas.
There are no reports of any protests on Tuesday.
Mr Utomi stressed that the protests should be peaceful and within the law.
'No legal challenge'
European Union observers say the elections fell far short of international standards and "cannot be considered to have been credible".
One Nigerian newspaper on Tuesday said their country had become a laughing stock.
"This is not the kind of Nigeria we dreamed of," said the independent daily, The Nation. The EU says at least 200 people have died since campaigning began.
Opposition candidate Muhammadu Buhari has told the BBC that he did not think Mr Yar'Adua would be sworn in as scheduled on 29 May but he did not give details of how this would be prevented.
"You have to be patient and see whether it will happen on the 29th of next month. But I very much doubt it," he said.
This should be the first time Africa's most populous nation replaces one elected civilian head with another.
Mr Buhari has, however, reportedly ruled out a legal challenge.
It took more than two years for his case, disputing the official results of the 2003 elections, to be finally rejected in court.
Mr Yar'Adua gained 24.6m votes, against 6.6m for his closest challenger, Muhammadu Buhari and 2.6m for vice-president turned opposition candidate Atiku Abubakar.
The lower house of the Nigerian parliament has directed one of its committees to investigate the allegations of widespread election fraud.
Committee members have seven days to submit their report.
Mr Obasanjo defended the organisation of the vote.
"No elections in the world will ever be regarded as perfect... You cannot use European standards to judge the situation in a developing country," he told the BBC.
Nigeria's biggest election monitoring group has said the presidential poll was so flawed that it should be scrapped and held again.
"In many parts of the country elections did not start on time or did not start at all," said Transition Monitoring Group chief Innocent Chukwuma.
Turnout was approximately 58%.
The US says it is "deeply troubled" by the weekend polls which it said were "flawed". A spokesman at the State Department said Washington hoped the political parties would resolve any differences over the election through peaceful, constitutional means.
Voter Donaman Atezan, 25, told the BBC News website that election material was delivered late to his polling station in the central Benue State, after most people had gone home.
"Thugs were then left alone to vote and each one of them voted for the PDP over and over as many times as the ballot papers were available," he said.
He said he tried to vote for an opposition candidate but the ballot paper was ripped from his hand.
Officials had struggled to deliver some of the 60m ballot papers to stations in time for the vote. They only arrived in the country on Friday evening.
The presidential poll was running alongside elections for the National Assembly and Senate.
Nigeria - one of the world's biggest oil producers - is of key strategic interest to both the West and the growing economies of the East.
But despite the country's huge oil wealth, tens of millions live in poverty.