Mr. Njoka said the criminal designation was based on evidence that the government had seen but did not plan to share, which he alleged may link the group to armed activity in the country.
“We’ve seen that they want to almost form a militia, and the government has to take action and deal with them before the government gets into that kind of situation,” Mr. Njoka said.
Mr. Njoka did not explain why the government had not previously spoken about the allegations of armed activity. “A number of reasons may not be shared with the media,” he said.
Salim Lone, an adviser to Mr. Odinga, said that the accusations were unfounded and that the group would not change its plans.
“We’re not paying any attention to such a declaration,” Mr. Lone said.
Earlier in the day, police officers and officials from the Communications Authority of Kenya descended on a broadcast transmission station in Limuru, about 18 miles outside Nairobi, the capital, and disconnected broadcasting equipment, according to Linus Kaikai, the chairman of the Kenya Editors Guild and the general manager of the television division at Nation Media Group, which owns NTV, one of the three channels disconnected.
Mr. Kaikai said that the authorities had disabled the equipment shortly before 9 a.m. “There was no explanation given,” he said.
Repeated calls to multiple officials at the Communications Authority were not returned.
The television blackout and the criminal designation seemed to add legitimacy to Mr. Odinga’s oath, which some observers had earlier dismissed as political theater.
“We’ve urged them to ignore it, to downplay it,” said one Western diplomat, who asked for anonymity because of the sensitivity of the matter. “It has no legal or practical effect, so the best solution here would be to just ignore the entire thing. But they’re not doing that.”
Mr. Njoka, the Interior Ministry spokesman, described the oath as an assault on legitimate government. He also said the government had “good reasons” for interrupting television broadcasts of the gathering.
“The government had to do what it did because the lives of Kenyans are more important than what you call freedom of the press or what might turn out to be an inciting broadcast,” he said.
When their transmissions were interrupted, the private broadcasters Citizen TV, KTN and NTV were showing people sitting peacefully in Uhuru Park in Nairobi. The stations continued to stream footage online, albeit with some interruptions, showing a growing but peaceful crowd.
Mr. Kenyatta summoned the owners of several media outlets to a meeting on Friday at the State House, the official residence of the president, and warned them against broadcasting any ceremony for Mr. Odinga.
Mr. Kaikai, the Editors Guild chairman, issued a statement denouncing the meeting, which he called a “brazen threat” that was “intended to intimidate the media from performing its rightful role of informing the public on matters affecting them.”
Hanningtone Gaya, chairman of the Media Owners Association of Kenya, attended the meeting on Friday, describing it as a “dressing down” at which media owners were “read the Riot Act,” according to Mr. Kaikai’s statement.
It is unclear what, specifically, prompted the government to dismantle the stations’ broadcasting equipment Tuesday morning.
“As far as I know, that has got absolutely no precedent,” said John Githongo, a longtime civil rights advocate, former government official and publisher of the nonpartisan political magazine The Elephant.
Mr. Githongo cited a government ban on live broadcasts during postelection violence in 2008, out of fear that bloodshed on television could incite further violence. Though live news broadcasts of the violence were banned, stations continued to transmit programming.
“I am not certain there is any precedent for this since the advent of multiparty politics in 1991,” Mr. Githongo said.
In November, the government considered imposing restrictions on news outlets after television stations broadcast a six-hour standoff between Mr. Odinga’s supporters and the police. Viewers watched as supporters of the opposition marched across Nairobi pursued by police officers who fired water cannons and tear gas and, according to witnesses and medical workers, live bullets. (The police have denied using live ammunition.)
At that time, Mr. Njoka told The New York Times that the broadcasts could be considered incitement to violence and that the government was weighing whether to further restrict coverage of opposition demonstrations. But no moves against the news media were made until Tuesday.
By 3 p.m. Tuesday, Mr. Odinga had taken an oath as the “people’s president” and had given a short speech in front of a crowd of thousands denouncing Kenya’s “electoral autocracy” and “election stealing.” The park emptied immediately after he spoke.
David Aduda, a veteran Kenyan journalist, said the government’s interruption of private broadcasts had “exploded” tensions that were building between the media and the government for months. Local journalists have complained privately that the government began interfering with coverage last year, as the political campaign season kicked off, but few wanted to go on the record about political interference in their work.
“There’s no doubt anymore that the government is out to cripple the media,” Mr. Aduda said. “Previously, you would hear of anecdotal evidence here and there, but there was no concrete evidence. Now, here we have it.”
“There is no justification for this,” he added, speaking of the broadcast disconnections. “This shows that we have a very intolerant government that does not respect media freedom, and for that reason, the media have every reason to keep fighting for every space to be able to operate according to the law.”
While Mr. Njoka, of the Interior Ministry, suggested that the television blackout was meant to maintain calm, Mr. Githongo of The Elephant said it could have the opposite effect.
“I’m getting calls from my relatives up country, who obviously know this is happening, and they’re getting bits from social media and telephone calls talking about mayhem in Nairobi, about a police crackdown — things that are not happening,” Mr. Githongo said.
“That’s what happens” with a blackout, he said. “The quality of the information goes down.”