LONDON/KATHMANDU British police charged a Nepali army colonel on Friday with two counts of torture during the Himalayan nation's decade-long civil war, despite the Nepali government's demanding his immediate release.
Nepal summoned the British ambassador earlier on Friday to express its "strong objection" to Kumar Lama's detention.
Rights groups accuse both the security forces and former Maoist rebels of committing abuses including torture during the conflict that killed more than 16,000 people.
The Maoists ended the conflict in 2006 under a peace deal with the government, won elections four years ago and are now heading a coalition ruling the young Himalayan republic.
London's Metropolitan Police said it had arrested Lama, 46, in the southern town of St. Leonards-on-Sea and charged him with committing acts of torture in 2005.
Media reports said he was detained while on vacation from a U.N. mission in Sudan.
The police statement accused Lama of intentionally inflicting "severe pain or suffering" on Janak Bahadur Raut between April 15 and May 1, 2005, and on Karam Hussain between April 15 and October 31, 2005.
Lama is due to appear in court in London on Saturday.
"We express strong objection to this mistake and urge that it be corrected ... and Lama be released," Foreign Minister Narayan Kaji Shrestha told reporters in Kathmandu after the colonel's arrest.
Human Rights Watch said the arrest sent a warning to those accused of serious crimes that they cannot hide from the law.
"The UK's move to arrest a Nepali army officer for torture during Nepal's brutal civil war is an important step in enforcing the U.N. Convention against Torture," Brad Adams, Asia director of Human Rights Watch, said in a statement.