LISBON (Reuters) - Police from Portugal's National Anti-Terrorism Unit arrested 20 ultra-nationalists on Tuesday in an operation that involved searches across the country as part of an investigation into attempted murder and other hate crimes.

The Lisbon District Prosecutor's office said in a statement those detained were part of the Hammerskin Nation, a neo-Nazi skinhead group based in the U.S. city of Dallas. "In our country they seek, among other things, to expel and prevent the entry of all ethnic minorities to Portugal," it said.

The Portuguese Hammerskin group could not immediately be contacted for comment.

The crimes being investigated took place between 2013 and 2015 and include at least one attempted murder, robbery, and religious and sexual discrimination, prosecutors said.

Police said some of those detained were suspected of involvement in an attack on a group of anti-fascist communist activists in September 2015 following an anti-immigration rally in downtown Lisbon staged by ultra-nationalist groups.

The police operation was the largest against neo-Nazi gangs since the 2007 arrest of 30 ultra-nationalists and their leader Mario Machado, who was sentenced to seven years in jail in 2010.

Attacks on immigrants and refugees are rare in western Europe's poorest country, which attracts fewer newcomers than richer countries like Spain or France but has large minorities rooted in its African ex-colonies as well as Brazil and India.

(Reporting by Andrei Khalip; Editing by Catherine Evans)