Ugandan police on Thursday arrested the prime minister of the traditional Rwenzururu kingdom, whose monarch is facing treason charges for allegedly seeking to overthrow the government, police said.

Police spokesman Suwed Manshur said that the king's number two, Johnson Thembo Kitsumbire "was arrested this afternoon (Thursday) but I am not in position to give further details".

The arrest comes a day after King Charles Wesley Mumbere and 151 others were charged with treason. The king is also facing charges of terrorism, aggravated robbery, attempted murder and murder.

Mumbere was arrested in late November during a violent police crackdown on his palace which left at least 87 dead.

"The state claims my clients overtly or covertly uttered statements with political intentions to overthrow the government or promote their own political agenda," said lawyer Evans Ochieng.

He said his clients denied the "trumped up" charges.

There were no further details of the charge, however the government has accused Mumbere of organising a militia from his palace with the aim of creating an independent state straddling Uganda and the neighbouring Democratic Republic of Congo.

The traditional kingdom in western Uganda has a history of separatist leanings dating back to independence. The region where it is based is an opposition stronghold that has long complained of marginalisation from Kampala.

International rights groups condemned the crackdown by security forces on the palace, with Amnesty expressing alarm at what "appears to be shocking examples of unlawful killings and a complete disregard for human rights during the arrests".