Local media say over 6,000 personnel from the state police and the elite General Service Unit have been involved in the operation that started on Friday, focusing on the Somali-dominated district Eastleigh of Nairobi.

The clampdown follows a series of attacks by suspected Islamists and sympathisers of the Al-Qaeda-linked Shebab rebels.

Those arrested have been subjected to identity checks, with many being held in police cells or a football stadium in Nairobi’s Kasarani district for further checks, officials and human rights activists said.

“We have arrested almost 4,000 people in this operation,” Kenya’s Interior Minister Joseph Ole Lenku said on the ministry’s Twitter account on Monday night.

The UN’s refugee agency said it was “concerned at the wave of arrests that have taken place during the weekend in Nairobi”, and demanded that it be allowed access to those detained.