Forty Ethiopians were arrested in a house in Kayole on Thursday for being in the country illegally.
Njiru subcounty commander Patrick Mwamba said they received a tip off from the public that the men were in the house in Maana area.
The men, aged between 10 and 25 years, could not speak English or Kiswahili, but told police through a translator that they were headed for South Africa.
They said they had arrived from Ethiopia on Monday, having paid an agent who was organising the trip. Police said they had no documentation from relevant immigration authorities.
Police are looking for the agent as they investigate the matter.
The men are currently in Kayole police station and will be taken to Makadara Law Courts.