The Metropolitan Police said officers from its war crimes team had arrested a 42-year-old man at an address in Leeds on Tuesday on suspicion of offences contrary to section 51 of the International Criminal Court Act 2001.
The force said the arrest was part of an ongoing investigation into alleged atrocities that occurred during the civil war in Sierra Leone between 1994 and 2000.
Officers also carried out a search of the property while the man was taken to a police station in Leeds before being released under investigation.
The civil war in Sierra Leone broke out in 1991 when the Revolutionary United Front (RUF) began a campaign against President Joseph Momoh, capturing towns on the border with Liberia.
The charity Action Aid said the west African country saw a third of its population displaced and tens of thousands killed during the 11-year conflict.
US-based non-government organisation (NGO) Human Rights Watch has said the conflict was characterised by “extreme brutality and widespread human rights abuses against civilians”.