More than five million telephone subscribers in Nigeria were cut off when protesters attacked an exchange of India-owned Airtel, one of the west African nation's biggest telecom firms, it said Thursday.

The protesters, members of the Nigeria Labour Congress (NLC), the nation's central labour movement, were protesting against the alleged casualisation of workers in Airtel and the dismissal of 3,000 employees, charges denied by the company.

"Some elements purporting to be members of NLC have shut down 15 of our showrooms across the country and also attacked our switch in Abuja, thereby cutting off over five million Nigerians from our network in the Federal Capital Territory (FCT) and parts of northern Nigeria," Airtel said in a statement.

"We appeal to our customers in the affected areas to please bear with us as we work with the Nigeria police to regain access to our switch and restore services."

NLC acting secretary general Owei Lakemfa told AFP that the unionists took the action against Airtel "after it sacked 3,000 workers at the end of September, and for refusing to pay some allowances it agreed with the workers."

"Since they said they wanted to shut down some parts of their operations, which they said led to the the sack, all we did was to help them quicken the process," Lakemfa said.

Airtel in the statement denied either hiring casual workers or dismissing the 3,000 employees as reported in some Nigerian newspapers.

"Airtel does not employ casual workers in any part of our organization... Airtel did not sack any employee," it said.

"The employees being referred to work for Tech Mahindra and Spanco, who provide us call centre services," it said.

India-based Bharti Airtel Limited is a leading global telecommunications company with operations in 19 countries across Asia and Africa.

The company boasts 233 million customers worldwide.