Three of Kenya's biggest television stations will remain off-air until further notice because they had planned to broadcast the opposition leader's unofficial "inauguration" in Nairobi.
Interior Minister Fred Matiang'i told reporters they would stay closed until investigations were complete.
Shortly after he spoke, opposition MP Tom Kajwang was arrested.
Mr Kajwang played a prominent role in Tuesday's ceremony naming Raila Odinga as "the people's president".
The outspoken MP had donned a lawyer's robe and wig, to the amusement of thousands of cheering opposition supporters in Uhuru park, in the capital Nairobi.
It is unclear what charges he will face.
The Interior Ministry said in a statement circulating on social media that broadcasting the event - described as an attempt to "subvert or overthrow" the government - "would have led to the deaths of thousands of innocent Kenyans".
As a result, NTV, KTN and Citizen TV were taken off-air at around 09:10 (06:10 GMT) on Tuesday.
Kenyan journalists denounced the move as outrageous and in a statement called for "respect of the constitution" and an end to the "unprecedented intimidation of journalists".
The regulatory body, the Media Council of Kenya, described the recent turn of events as "the greatest threat and assault on freedom of expression and media in Kenya's recent history", saying there was no need to keep the media houses closed while an investigation was ongoing.
"This shutdown erodes the gains so far made in developing a free and responsible media industry and should never happen in a robust democracy that Kenya boasts of," the organisation said in a Facebook post. "As guaranteed by the Kenyan Constitution, media freedom should be guarded jealously at all times."
But the Interior Ministry said it was a "serious breach of security".
Mr Matiang'i told reporters: "As a result of what happened yesterday, we have commenced investigations targeting individuals. Media houses will remain closed until we complete investigations."
Mr Odinga held his own "inauguration" after claiming he was the rightful winner of last year's election.
President Uhuru Kenyatta was sworn in for a second term last November. He won an election re-run in October, but Mr Odinga boycotted it.
Mr Kenyatta's initial victory was overturned by the Supreme Court, citing "irregularities and illegalities".
The inauguration event in Uhuru park was attended by thousands of supporters, and went off peacefully.
Citizen TV live streamed the event on its website, and on YouTube and Facebook.