Nigerian police have arrested three men allegedly contracted to print as many as five million fake voters' cards ahead of April's presidential and parliamentary elections.

We are still looking for the man who paid the business centre to print the documents Police spokesman Emmanuel Ighodalo

The operation, in which police also seized 500,000 faked voting documents, took place in Lagos following a tip-off a week ago but was only announced on Friday.

Earlier this week, the Independent National Electoral Commission (Inec) was reported to have discovered two million fake registration cards across the country.

The BBC's Dan Isaacs in Lagos says that if successful, the latest vote-rigging scheme could easily have swung the elections.

He said the controversy will provide plenty of ammunition for defeated candidates to cry foul.

The April poll will be first to be held by a civilian government for some 20 years.

The last occasion, in 1983, was widely regarded to have been deeply flawed, and was followed after only a few months by a military takeover.

Incumbent President Olesegun Obasanjo is facing stiff competition from another former military head of state, Muhammadu Buhari.

Elections will also be taking place to elect governors, senators and members of federal and state parliaments.

Retired general

Police said the three men, including the manager of the business centre where the documents were being printed, would remain in police custody during the investigation.

The men told police that the 500,000 fake voters' cards that were seized were there first batch that they had printed.

Police said they were looking for the man who had placed the order for 100 million naira (about $800,000).

"We are still looking for the man who paid the business centre to print the documents," police spokesman Emmanuel Ighodalo said.

"We don't know what political party he belongs to, but one of the suspects has said he was a retired military officer."

Mr Ighodalo said the seized voters' cards looked authentic.

Troubled start

"This was an eye-opener. We are now working hard and using our intelligence sources to see if more of these activities are going on," he said.

Earlier this week sources from Inec hinted that the electoral body had uncovered more than two million of such forged registration cards nationwide, representing 3.3% of the projected registration figure of about 59.5 million, according to Nigeria's Guardian newspaper.

Many voters have been disenfranchised

Inec also announced a fresh round of voter registration would take place next week.

The raid and arrests come in the wake of primary elections which human rights groups and NGOs have said were marred by bribery.

"There was widespread bribery of delegates with sacks stuffed with money to influence their votes," the Transition Monitoring Group was quoted as saying by Reuters news agency.