Many Nigerians arrived in Sierra Leone as peacekeepers
Immigration officers in Sierra Leone have started a manhunt for illegal immigrants living in the country.
Chief immigration officer Alpha Kolifa Koroma told the BBC the illegal immigrants were committing crimes such as drug dealing and people trafficking.
Several hundred have been arrested and four deported in the last week.
The BBC's Umaru Fofana in Sierra Leone says it is believed there are tens of thousands of foreigners, mainly from Nigeria, living there without records.
Our correspondent says the number of Nigerians has soared dramatically since the mid 1990s because of the presence of Nigerian peacekeepers in Sierra Leone.
Nigeria led a West African force that intervened during Sierra Leone's brutal civil war.
The Nigerian soldiers later joined the UN peace mission, which withdrew in 2005 - three years after the conflict officially ended.
Our reporter says the news of the crackdown has prompted fear among the large Nigerian community - many fearing it may prompt xenophobic attacks.
Mr Koroma said the operation had been launched because the country has become "plagued by so many anti-social activities orchestrated by alleged foreign nationals".
"There are all kinds of drug trafficking, child trafficking orchestrated by these people and these are relatively new criminal activities in the country," he told the BBC's Network Africa programme.
He urged foreigners to come to the immigration headquarters to legalise their status. Two Algerians and two Nigerians have been deported since the crackdown began.
Citizens of members of the regional body, the Economic Community Of West African States (Ecowas), do not require visas to stay in other member states.
But after 90 days, Ecowas citizens should register if they wish to stay longer.
Our correspondent says thousands of Nigerians live all over Sierra Leone, many running shops in business district of the capital, Freetown.
But many Sierra Leoneans regard them with suspicion as they believe they are involved in crime, especially drug dealing or con tricks - known as 419 scams, he says.
Nigerians in Freetown have voiced disquiet about the operation - some telling the BBC that as Ecowas citizens, they should have the right to remain, others fearing the move will entrench resentment towards them.