Former Kenyan vice president Kalonzo Musyoka has said his home was targeted with gunfire and a stun grenade in an assassination attempt.
The incident comes as a government shutdown of Kenya's top three TV stations continues after they showed the mock inauguration of opposition leader Raila Odinga as "the people's president".
The first arrest has been reported as part of a government investigation into the event's "conspirators".
Mr Musyoka said his home was attacked hours after his police security was withdrawn and he was blocked from attending Mr Odinga's mock ceremony in Nairobi.
The "inauguration" was meant as a protest against President Uhuru Kenyatta's new term after months of deadly election turmoil.
I wish to thank the people of Kenya for the mandate they have given us and for their steadfast confidence in us. You came from all corners of the republic to witness my inauguration and it was good to see you out in millions. pic.twitter.com/U7oCAhsHPI— Raila Odinga (@RailaOdinga) January 30, 2018
Mr Musyoka was to take the oath as Mr Odinga's deputy as the opposition leader declared himself "the people's president".
Police spokesman Charles Owino confirmed that a stun grenade was hurled at Mr Musyoka's home in the leafy Karen suburbs and two shots were fired from a rifle.
"The motive was clearly political," Mr Musyoka said, describing the 1am blast as "shocking".
He said he felt vulnerable after his police security was withdrawn and hired a private security firm. "I knew we were vulnerable but I didn't think they would strike that fast," he said.
No-one was hurt in the incident.
Kenya's interior minister, Fred Matiangi, announced an investigation into "conspirators and facilitators" of Tuesday's event. The government has outlawed the opposition's National Resistance Movement, with Mr Matiangi declaring it an organised criminal group. Membership in such a group can lead to imprisonment up to 10 years under Kenyan law.
The man who swore in Mr Odinga has been arrested, his lawyer said. Opposition legislator and lawyer TJ Kajwang has been taken to Nairobi's criminal investigations department for questioning.
The interior minister also said the top three TV stations and some radio stations will remain shut down while being investigated for their alleged role in what he called an attempt to "subvert and overthrow" Kenyatta's government.
Mr Matiangi claimed that the media's complicity in Tuesday's ceremony would have led to the deaths of thousands of Kenyans.