Joe Ajaero, President of the Nigeria Labour Congress (NLC), claims he was quizzed for about 15 hours regarding the #EndBadGovernance countrywide rallies in August.
The union leader was arrested on his way to the United Kingdom for a Trade Union Congress (TUC) session. He was released on bail at about 11 p.m.
The NLC leader informed Channels TV that his travel passport had been taken by the secret police.
He said some police officers came to the DSS facility to grill him.
Ajaero said he was questioned over alleged terrorism financing involving a British national, Andrew Wynne, who is wanted by Nigerian authorities.
The TUC, global rights group Amnesty International, and human rights activist Femi Falana, among others, criticized the Federal Government for clamping down on the labour leader and demanded his immediate and unconditional release.
Ajaero met the police in late August to face questions over allegations of terrorism financing. He was scheduled to meet the police again before his arrest.
