The main labour union, Nigerian Labour Congress (NLC), which represents millions of workers across most sectors of Africa's largest economy, said on Friday that it will stage a warning strike from Tuesday to protest at the cost of living and then plans indefinite action later this month.

An election tribunal meanwhile is due to deliver its ruling on Wednesday on petitions by the two main opposition candidates to President Bola Tinubu's election victory in February.

The Department of State Services (DSS) said it has uncovered plots by certain politicians to mobilise students and youth groups to stage violent protests in the country to discredit the government over socio-economic matters, but did not provide further details or name any politicians.

It said its "intelligence reports have indicated that the plotters include certain politicians who are desperately mobilising unsuspecting student leaders, ethnic-based associations, youth and disgruntled groups for the planned action.

It added that "the service has identified the ring leaders of the plot as well as sustained monitoring around them in order to deter them from plunging the country into anarchy."

Tinubu, who has embarked on Nigeria's boldest reforms in decades, has been under pressure from unions to offer relief to millions of households and small businesses after he scrapped the subsidy that had kept petrol prices cheap but cost the government $10 billion last year.

The NLC said its actions on Tuesday and Wednesday will "demonstrate our readiness" for an indefinite strike due to start on Sept. 21.

Unions staged a strike a month ago over the cost of living but suspended their action following an offer of talks with the government, but those talks did not result in concessions sufficient to placate organised labour.

(Reporting by Camillus Eboh; Writing by Chijioke Ohuocha; Editing by Susan Fenton)

By Camillus Eboh