The location of Adams Oshiomhole, president of the Nigeria Labour Congress (NLC), remained a mystery more than six hours after he was dragged out of Abuja airport's domestic terminal by armed plainclothes officers.
Nigeria's domestic security agency said that the arrest had been the result of a misunderstanding and that Oshiomhole had been swiftly released.
But the firebrand leader's allies insisted that they were unable to locate him and claimed that he was being held incommunicado at a secret location.
Oshiomhole's NLC has called for Nigerians to stay at home from Monday as part of a general strike to protest rising petrol prices under President Olusegun Obasanjo's ambitious programme of economic reform.
A statement from Nigeria's secret police, the State Security Service, confirmed that Oshiomhole had been picked up at the airport.
"Mr Adams Oshiomhole had some misunderstanding with field operatives at the local terminal of Nnamdi Azikwe Airport, Abuja, as a result of which he was taken to the sub-office of the service at Asokoro," the statement said.
"On learning of the incident the director general of the SSS personally went to the sub-office at about 10.00 am and resolved the matter. Thereafter, the NLC president left the premises at 10.05 am," it said.
But union leaders dismissed this explanation.
"It is bad enough that the NLC President, Mr. Adams Oshiomhole, had been arrested, it is now even worse that this administration is disputing the fact of his continuing detention," said NLC secretary general Salihu Lukman.
"There are now legitimate concerns about his life and safety," he added.
Earlier the unions had accused the SSS of using strongarm tactics.
"Mr Oshiomhole was abducted by a team of operatives of the State Security Services numbering over 15, who overpowered him, wrestled him to the ground and bundled him into a standby Peugot 504 station wagon," a NLC statement said.
"In the process his jacket was torn and he sustained bruises caused by rough handling and the impact of the several falls he sustained before being half-dragged, half-carried into the vehicle," it continued.
The NLC's national mobilisation officer, Denja Yacqub, said: "We suspect his arrest is a means of intimidating us. His absence will not affect the strike. We will go ahead."
However, Femi Falana, the chairman of the West African Bar Association and one of the NLC's lawyers, said at 4.30 pm that he had heard that Oshiomhole had been released "two hours ago" but had not yet spoken to him to confirm this.
The NLC, which represents most of Nigeria's trade unions, is due to launch a general strike from Monday in protest against the latest in a series of increases in retail petrol and diesel prices.
In doing so, it will once more come into conflict with Obasanjo's government, which sees the deregulation of Nigeria's subsidised fuel sector as a key plank in an ambitious programme of economic liberalisation.
World oil markets are watching the preparations for the strike closely, fearful it could disrupt Nigeria's exports of crude oil at a time when prices have already soared to record levels.
Nigeria is Africa's biggest oil producer, exporting between 2.3 and 2.6 million barrels of sweet light crude every day. Previous strikes have not, however, done much to interrupt the flow.
