A 46-year-old man has been arrested on suspicion of torture carried out during the civil war in Nepal, Scotland Yard said.
Counter-terrorism officers held the man in St Leonards-on-Sea, East Sussex, and he is being questioned at a police station in the county.
The unit is responsible for investigating alleged war crimes and human rights abuses, and the arrest relates to claims that acts of torture were committed in Nepal in 2005.
Officers are searching a residential address in St Leonards-on-Sea, the Metropolitan Police said.
At least 13,000 people died during the conflict in Nepal, which lasted for a decade until 2006.
The United Nations said that up to 9,000 serious human rights violations could have been committed during the war, including forced disappearances, torture and executions.
It is understood that the man was a supporter of the then-government of Nepal during the conflict.
A third party made allegations against him to counter-terrorism teams in the UK, who then began their own investigation.
If charged, the man could face prosecution in Britain because torture is an offence of universal jurisdiction, and he has been living in the UK for some years, Scotland Yard said.
However officers would need to gather corroborating evidence in this country to support any charge. The man was arrested on suspicion of torture contrary to Section 134 of the Criminal Justice Act 1988.