Nigerian unions have called a two-day strike to protest last month's elections, which they say were marred by widespread voter fraud.

Trade Union Congress (TUC) secretary-general John Kolawale says the strike will take place on May 28 and May 29.

He says the dates were decided in conjunction with another labor organization, the Nigeria Labor Congress, and civil society groups. The strike will coincide with the May 29 inauguration of President-elect Umaru Yar'Adua.

On Wednesday, Nigeria's Inspector General of Police, Sunday Ehindero, warned against any form of protest not approved by the police. He said the police will take the necessary action to disband any unauthorized marches or gatherings.

Last month, Umaru Yar'Adua, of the ruling People's Democratic Party, was declared winner of Nigeria's presidential election in a vote that many observers described as flawed.

Opposition groups have rejected the election results and say they will challenge the outcome in court.

Some information for this report was provided by AFP, AP and Reuters.