Kenyan security forces have rounded up thousands of Somalis or Kenyans of Somali origin in a huge crackdown on suspected Islamists blamed for a string of attacks, leaving many languishing in a football stadium.
The operation, reportedly involving more than 6,000 police and elite officers, started on Friday and has focused on Eastleigh, an ethnic Somali-dominated district of the Kenyan capital known as "Little Mogadishu".
The crackdown follows a spate of attacks in Kenya by suspected Islamists and sympathisers of Somalia's Al-Qaeda-linked Shebab rebels, including last September's deadly siege of a Nairobi shopping mall.
Three blasts in Eastleigh on March 31 that killed six people appear to have triggered the latest police crackdown.
Those arrested have been subjected to identity checks, with hundreds and maybe thousands 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.
"For the last few months we've had heightened insecurity. Time has come for a mop up to restore order."
A police source said 1,600 of those arrested since Friday had been released so far, although it was impossible to verify how many people are still being held inside the stadium as journalists were denied access.
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.
"UNHCR has sought access for itself and its partners to the detained refugees and asylum-seekers. This access will allow UNHCR to properly identify refugees, asylum-seekers and others of concern," the agency said in a statement.
The UN agency said it understood Kenya's security concerns, but urged security forces "to uphold the rights of all those arrested and to treat them in a humane and non-discriminatory manner."
- Deportees 'will join Shebab' -
Residents of Eastleigh accused the police of breaking into houses without a search warrant and of indiscriminately arresting minors and elderly people.
Rights groups often accuse the Kenyan security forces of being too heavy-handed during such crackdowns, with the result that many Muslim youths turn against the Kenyan government.
The authorities argue they are obliged to crack down on Somalis who are in the country illegally.
Those charged with illegally staying in Kenya will be deported, a Kenyan official said.
Paramilitary police spokesman Masoud Mwinyi said 3,000 people have been screened so far, 69 people have been charged with various offences and a further 467 have been detained for investigation.
The Somali ambassador to Kenya, Mohamed Ali Nur said that those with valid visas or proven refugee status had been released.
Last September Shebab Islamists laid siege to Nairobi's upmarket Westgate shopping mall, killing at least 67 people.
Nairobi has been one of the prime targets for smaller attacks attributed to Shebab sympathisers ever since Kenya sent troops into Somalia to fight the extremists in October 2011.
"If Kenya deports all the young men I am sure that will lead to more of them joining the Shebab," Ali, a 28-year-old resident of Eastleigh told AFP.