Four men have been killed after Sierra Leone's navy acted against a pirate attack, police say.
A group of pirates from neighbouring Guinea opened fire on a Chinese fishing vessel, the Shanghai Three, police spokesman Ibrahim Samura told the BBC.
He said following the intervention of the navy, two pirates were shot dead and two men believed to be Guinean soldiers drowned.
A BBC reporter says pirate attacks are rare in West Africa, outside Nigeria.
At least four alleged pirates, from Guinea and Sierra Leone, were arrested.
"The pirates boarded the vessels... and put the crew under gunpoint. We responded to a distress call from one of the vessel's captains and a three-hour battle ensued," Mr Samura said.
The fishing boat's Chinese and Sierra Leonean crew was unharmed.
BBC West Africa correspondent Will Ross says there have been warnings that piracy is growing in the region and the small under-resourced navies need help to counter the threat.
The fact that West Africa is being used as a staging post for Latin American cocaine is adding to the insecurity.
Our correspondent says there have also been several oil discoveries in the region and there is a need to improve the protection of these offshore installations.
Fishermen who complain that illegal trawlers are depleting fish stocks would also welcome better policing of the West African coastline, he says.
The waters off Africa's east coast are the most dangerous in the world. Earlier this month, Somali pirates seized a Saudi super-tanker carrying 2m barrels of oil.