Police on Tuesday arrested two people linked to Islamic State (ISIS) and seized bomb-making materials from them at Nairobi's Kangemi estate.
The duo was said to be planning a retaliatory attack following the arrest of the Islamic State planner Mohamed Abdi Ali.
Ali, a medical student who was interning at a Kenyan hospital, was arrested on April 29, 2016.
His wife and her friend were also arrested later on and are in the custody of Uganda police.
"The arrest of the two has foiled terror attacks with explosives and other weapons that were planned for Nairobi and Mombasa," the police said.
Inspector general Joseph Boinett, in a statement signed by George Kinoti, said thorough investigations led to the arrest of Kiguzo Mwangolo Mgutu and Abubakar Jillo Mohamed.
In one house at Kangemi, police found materials that were to be used for the manufacture of an improvised explosive device.
"These materials included nails, ball bearings, batteries, electrical wire, fertilizer, cell phones and other explosive substances which have been taken for forensic analysis," police said.
Initial investigations showed the two men helped to write a document that has been circulating online, supporting Islamic States leader Abu Bakr al-Baghdadi, the police said.
“Explosive experts have indicated that these are the materials terrorists typically use in the making of IEDs,” Kinoti said.
The statement said that bows and poisoned arrows were also recovered from the suspect’s house.
Kinoti said the suspects were planning to escape to Syria or Libya after carrying out terror attacks targeting public transport users.
Kinoti warned that the suspects were making the plans without their parents’ knowledge.
He asked parents and guardians to be vigilant and monitor their children’s activities as well as their friends.
“Similarly, we advise members of the public to be more vigilant while in public transport. Identify persons who may behave suspiciously and report to the police immediately,” said Kinoti.
He also called on public transport operators to be extra vigilant.
The arrests happened in the wake of Kenya facing a series of attacks in the past few years in which hundreds have died.
These were launched by the al Qaeda-aligned Somali Islamist group al Shabaab.
This month, however, police said they had arrested an Islamist militant who was plotting attacks and who they said was linked to Islamic State.
Kenya's focus on dismantling al Shabaab cells may have allowed sympathisers of Islamic State to slip under the radar, experts say.
But they say it is not clear whether such sympathisers had formal links to Islamic State in Syria or Iraq, or were simply offering allegiance.