Nigerian opposition and civil society groups are to join trade unions at May Day rallies to protest against recent presidential and state elections.
The country's head of police said unauthorised demonstrations would be forcibly broken up with teargas.
The ruling People's Democratic Party's Umaru Yar'Adua was declared the winner of the 21 April presidential election.
Monitors have said both the state and presidential polls were rigged but Mr Yar'Adua says his victory was fair.
Police crackdown
Inspector General of Police Sunday Ehindero said May Day rallies were allowed at designated grounds, but police would crack down on those marching without a permit.
"Any procession that takes place without a police permit will be dispersed forcefully, it will be tear-gassed and if greater force is required, we will use it," Mr Ehindero said.
The main opposition leader - Muhammadu Buhari of the All Nigeria People's Party - is expected to attend one of the rallies.
On Monday, top election observer Emma Ezeazu was arrested by state security officials, who accuse him of inciting violence.
The agents from the State Security Service (SSS) raided the Alliance for Credible Elections (ACE) office and confiscated placards being readied for the protests, ACE says.
"They said they didn't need a warrant to arrest him," ACE spokesman Odoh Okenyodo told the BBC's News website.
"They said they caught us in the act of producing placards for tomorrow's mass protests.
"But this will not break our will. International and local observers all agree that the recent general elections in Nigeria were terribly rigged.
"We were only preparing those placards to join in the peaceful protests called tomorrow by the central labour union" he says.
Fears
Mr Ezeazu has said the election was a "charade".
"People were forced to thumb-print in front of the PDP agents and then they took the ballot and put it in the ballot box," he told the BBC.
Some 200 people were killed in election violence, according to European Union observers.
The BBC's Alex Last in Lagos says the opposition has been reluctant to call for street protests because it fears they could spiral out of control.
Many question whether people will be willing to potentially risk their lives to speak out against the polls, our correspondent says.
Opposition parties have called for the annulment of the elections.
But the government has rejected criticism of the elections, saying it was intended to cause a coup.
When President Olusegun Obasanjo hands over power on 29 May it will be the first transition from one civilian leader to another in what is Africa's most populous nation.
PHOTO CAPTION
Nigeria's president-elect Umaru Yar'Adua addresses a rally in Abuja, March 29, 2007. (Reuters)
BBC