A large shipment of weapons has been seized by Nigeria's State Security Service at the port in Lagos city.
The secret police say they intercepted 13 containers, some of which had rocket launchers and grenades and other explosives hidden under tiles.
The authorities have said they cannot say who owned the containers or their intended destination.
But correspondents say the discovery has increased fears of possible violence during next year's elections.
Intelligence agencies say they are still sifting through the weapons and they also included artillery rockets, mortar rounds and rocket launchers.
The BBC's Caroline Duffield in Lagos says the type of weapons described makes this a major departure from previous arms hauls.
Only a small part of the arsenal was shown to the media, she says, but it was enough to worry security experts.
They identified the artillery rockets as Norinco rockets - a type used by the Taliban in Afghanistan - suitable for high-intensity warfare.
Official sources told the BBC the shipment came from Iran.
But the National Security Adviser, Andrew Owoye Azazi, told journalists the authorities were still establishing the origin of the weapons and their intended destination.
"At this point, the only thing we can say is that we have some armaments we have discovered at the port... Let's not jump to conclusions," Mr Azazi said.
Officials told the BBC there had been some arrests, but gave no further details.
Security has been stepped up in Lagos since the bombings on 1 October during celebrations of the 50th anniversary of independence.
A militant group from the oil-producing Niger Delta region claimed responsibility for the blasts, and it has threatened to strike again.
Our correspondent says that group has never used heavy weapons of the type just seized in its attacks, and there are fears it might be changing its tactics.
But there is also insecurity in Nigeria's northeast involving the radical Islamist group, Boko Haram -- and this arms find is also prompting fears of a new uprising there, our correspondent adds.